Remove to_cstring() naming convention ambiguity

Add a trait for str.strdup() to replace to_cstring() which avoid the
signature ambiguity with .to_string().

Also instruduce CString::yolo() as a shortcut to
CString::new().unwrap() and use it whenever the variable does can be
deallocated by going out of scope.  This is less error prone.

Use some Path.to_c_string() functions where possible.

src/dc_configure.rs

@@ -1102,14 +1102,14 @@ unsafe fn moz_autoconfigure(
         tag_config: 0,
     };
 
-    let url_c = to_cstring(url);
+    let url_c = url.strdup();
     let xml_raw = read_autoconf_file(context, url_c);
     free(url_c as *mut libc::c_void);
     if xml_raw.is_null() {
         return None;
     }
 
-    moz_ac.in_emaillocalpart = to_cstring(&param_in.addr);
+    moz_ac.in_emaillocalpart = param_in.addr.strdup();
     let p = strchr(moz_ac.in_emaillocalpart, '@' as i32);
 
     if p.is_null() {
@@ -1166,7 +1166,7 @@ unsafe fn moz_autoconfigure_text_cb(
     let mut moz_ac: *mut moz_autoconfigure_t = userdata as *mut moz_autoconfigure_t;
     let mut val: *mut libc::c_char = dc_strdup(text);
     dc_trim(val);
-    let addr = to_cstring(&(*moz_ac).in_0.addr);
+    let addr = (*moz_ac).in_0.addr.strdup();
     dc_str_replace(
         &mut val,
         b"%EMAILADDRESS%\x00" as *const u8 as *const libc::c_char,
@@ -1306,7 +1306,7 @@ fn read_autoconf_file(context: &Context, url: *const libc::c_char) -> *mut libc:
         .send()
         .and_then(|mut res| res.text())
     {
-        Ok(res) => unsafe { to_cstring(res) },
+        Ok(res) => unsafe { res.strdup() },
         Err(_err) => {
             info!(context, 0, "Can\'t read file.",);
 
@@ -1322,7 +1322,7 @@ unsafe fn outlk_autodiscover(
 ) -> Option<dc_loginparam_t> {
     let current_block: u64;
     let mut xml_raw: *mut libc::c_char = 0 as *mut libc::c_char;
-    let mut url = to_cstring(url__);
+    let mut url = url__.strdup();
     let mut outlk_ad = outlk_autodiscover_t {
         in_0: param_in,
         out: dc_loginparam_new(),