Remove unsafe CString::yolo from ffi

CString::yolo was still used in the ffi, this was an unsafe
transitional thing.  To remove it there were two choices: 1. make
errors in creating CStrings hard errors or 2. try and be as lenient as
possible.  Given the to_string_lossy() convention adopted in the ffi
this choose the lenient option and simply skips over embedded null
bytes, leaving the rest of the strings intact.

Thus now CString::new_lossy().  It's only used for .strdup() however
so no longer a public trait.

This also cleans up the public visibility of things in the strings.rs
file:

- Rename StrExt/OptStrExt traits to what they actually do: provide
  .strdup() -> Strdup/OptStrdup.

- dc_strdup() should be an implementation detail, replace all usages
  with Strdup.strdup() method.

- Only allow visibility inside the crate for all things.

- Reduce visibility to only the module for things not used in lib.rs.

deltachat-ffi/src/lib.rs

@@ -433,7 +433,7 @@ pub unsafe extern "C" fn dc_set_config_from_qr(
 pub unsafe extern "C" fn dc_get_info(context: *mut dc_context_t) -> *mut libc::c_char {
     if context.is_null() {
         eprintln!("ignoring careless call to dc_get_info()");
-        return dc_strdup(ptr::null());
+        return "".strdup();
     }
     let ffi_context = &*context;
     let guard = ffi_context.inner.read().unwrap();
@@ -1455,7 +1455,7 @@ pub unsafe extern "C" fn dc_get_msg_info(
 ) -> *mut libc::c_char {
     if context.is_null() {
         eprintln!("ignoring careless call to dc_get_msg_info()");
-        return dc_strdup(ptr::null());
+        return "".strdup();
     }
     let ffi_context = &*context;
     ffi_context
@@ -2406,7 +2406,7 @@ pub unsafe extern "C" fn dc_chat_get_type(chat: *mut dc_chat_t) -> libc::c_int {
 pub unsafe extern "C" fn dc_chat_get_name(chat: *mut dc_chat_t) -> *mut libc::c_char {
     if chat.is_null() {
         eprintln!("ignoring careless call to dc_chat_get_name()");
-        return dc_strdup(ptr::null());
+        return "".strdup();
     }
     let ffi_chat = &*chat;
     ffi_chat.chat.get_name().strdup()
@@ -2728,7 +2728,7 @@ pub unsafe extern "C" fn dc_msg_get_sort_timestamp(msg: *mut dc_msg_t) -> i64 {
 pub unsafe extern "C" fn dc_msg_get_text(msg: *mut dc_msg_t) -> *mut libc::c_char {
     if msg.is_null() {
         eprintln!("ignoring careless call to dc_msg_get_text()");
-        return dc_strdup(ptr::null());
+        return "".strdup();
     }
     let ffi_msg = &*msg;
     ffi_msg.message.get_text().unwrap_or_default().strdup()
@@ -2747,8 +2747,7 @@ pub unsafe extern "C" fn dc_msg_get_file(msg: *mut dc_msg_t) -> *mut libc::c_cha
             ffi_msg
                 .message
                 .get_file(ctx)
-                .and_then(|p| p.to_c_string().ok())
-                .map(|cs| dc_strdup(cs.as_ptr()))
+                .map(|p| p.strdup())
                 .unwrap_or_else(|| "".strdup())
         })
         .unwrap_or_else(|_| "".strdup())
@@ -2758,7 +2757,7 @@ pub unsafe extern "C" fn dc_msg_get_file(msg: *mut dc_msg_t) -> *mut libc::c_cha
 pub unsafe extern "C" fn dc_msg_get_filename(msg: *mut dc_msg_t) -> *mut libc::c_char {
     if msg.is_null() {
         eprintln!("ignoring careless call to dc_msg_get_filename()");
-        return dc_strdup(ptr::null());
+        return "".strdup();
     }
     let ffi_msg = &*msg;
     ffi_msg.message.get_filename().unwrap_or_default().strdup()
@@ -2768,13 +2767,13 @@ pub unsafe extern "C" fn dc_msg_get_filename(msg: *mut dc_msg_t) -> *mut libc::c
 pub unsafe extern "C" fn dc_msg_get_filemime(msg: *mut dc_msg_t) -> *mut libc::c_char {
     if msg.is_null() {
         eprintln!("ignoring careless call to dc_msg_get_filemime()");
-        return dc_strdup(ptr::null());
+        return "".strdup();
     }
     let ffi_msg = &*msg;
     if let Some(x) = ffi_msg.message.get_filemime() {
         x.strdup()
     } else {
-        dc_strdup(ptr::null())
+        "".strdup()
     }
 }
 
@@ -3098,7 +3097,7 @@ pub unsafe extern "C" fn dc_contact_get_id(contact: *mut dc_contact_t) -> u32 {
 pub unsafe extern "C" fn dc_contact_get_addr(contact: *mut dc_contact_t) -> *mut libc::c_char {
     if contact.is_null() {
         eprintln!("ignoring careless call to dc_contact_get_addr()");
-        return dc_strdup(ptr::null());
+        return "".strdup();
     }
     let ffi_contact = &*contact;
     ffi_contact.contact.get_addr().strdup()
@@ -3108,7 +3107,7 @@ pub unsafe extern "C" fn dc_contact_get_addr(contact: *mut dc_contact_t) -> *mut
 pub unsafe extern "C" fn dc_contact_get_name(contact: *mut dc_contact_t) -> *mut libc::c_char {
     if contact.is_null() {
         eprintln!("ignoring careless call to dc_contact_get_name()");
-        return dc_strdup(ptr::null());
+        return "".strdup();
     }
     let ffi_contact = &*contact;
     ffi_contact.contact.get_name().strdup()
@@ -3120,7 +3119,7 @@ pub unsafe extern "C" fn dc_contact_get_display_name(
 ) -> *mut libc::c_char {
     if contact.is_null() {
         eprintln!("ignoring careless call to dc_contact_get_display_name()");
-        return dc_strdup(ptr::null());
+        return "".strdup();
     }
     let ffi_contact = &*contact;
     ffi_contact.contact.get_display_name().strdup()
@@ -3132,7 +3131,7 @@ pub unsafe extern "C" fn dc_contact_get_name_n_addr(
 ) -> *mut libc::c_char {
     if contact.is_null() {
         eprintln!("ignoring careless call to dc_contact_get_name_n_addr()");
-        return dc_strdup(ptr::null());
+        return "".strdup();
     }
     let ffi_contact = &*contact;
     ffi_contact.contact.get_name_n_addr().strdup()
@@ -3144,7 +3143,7 @@ pub unsafe extern "C" fn dc_contact_get_first_name(
 ) -> *mut libc::c_char {
     if contact.is_null() {
         eprintln!("ignoring careless call to dc_contact_get_first_name()");
-        return dc_strdup(ptr::null());
+        return "".strdup();
     }
     let ffi_contact = &*contact;
     ffi_contact.contact.get_first_name().strdup()