Make stock strings rusty

This converts the stock strings API to be more safe-rust style.  The
API is kept roughly the same for now but moved to methods on the
context.

src/dc_securejoin.rs

@@ -1,3 +1,5 @@
+use std::ffi::CString;
+
 use mmime::mailimf_types::*;
 use percent_encoding::{utf8_percent_encode, DEFAULT_ENCODE_SET};
 
@@ -14,12 +16,12 @@ use crate::dc_mimeparser::*;
 use crate::dc_msg::*;
 use crate::dc_param::*;
 use crate::dc_qr::*;
-use crate::dc_stock::*;
 use crate::dc_strencode::*;
 use crate::dc_token::*;
 use crate::dc_tools::*;
 use crate::key::*;
 use crate::peerstate::*;
+use crate::stock::StockMessage;
 use crate::types::*;
 use crate::x::*;
 
@@ -797,22 +799,21 @@ unsafe fn end_bobs_joining(context: &Context, status: libc::c_int) {
 unsafe fn secure_connection_established(context: &Context, contact_chat_id: uint32_t) {
     let contact_id: uint32_t = chat_id_2_contact_id(context, contact_chat_id);
     let contact: *mut dc_contact_t = dc_get_contact(context, contact_id);
-    let msg: *mut libc::c_char = dc_stock_str_repl_string(
-        context,
-        35i32,
+    let msg = CString::new(context.stock_string_repl_str(
+        StockMessage::ContactVerified,
         if !contact.is_null() {
-            (*contact).addr
+            as_str((*contact).addr)
         } else {
-            b"?\x00" as *const u8 as *const libc::c_char
+            "?"
         },
-    );
-    dc_add_device_msg(context, contact_chat_id, msg);
+    ))
+    .unwrap();
+    dc_add_device_msg(context, contact_chat_id, msg.as_ptr());
     context.call_cb(
         Event::CHAT_MODIFIED,
         contact_chat_id as uintptr_t,
         0i32 as uintptr_t,
     );
-    free(msg as *mut libc::c_void);
     dc_contact_unref(contact);
 }
 
@@ -840,18 +841,17 @@ unsafe fn could_not_establish_secure_connection(
 ) {
     let contact_id: uint32_t = chat_id_2_contact_id(context, contact_chat_id);
     let contact = dc_get_contact(context, contact_id);
-    let msg: *mut libc::c_char = dc_stock_str_repl_string(
-        context,
-        36i32,
+    let msg = context.stock_string_repl_str(
+        StockMessage::ContactNotVerified,
         if !contact.is_null() {
-            (*contact).addr
+            as_str((*contact).addr)
         } else {
-            b"?\x00" as *const u8 as *const libc::c_char
+            "?"
         },
     );
-    dc_add_device_msg(context, contact_chat_id, msg);
-    error!(context, 0, "{} ({})", as_str(msg), to_string(details),);
-    free(msg as *mut libc::c_void);
+    let msg_c = CString::new(msg.as_str()).unwrap();
+    dc_add_device_msg(context, contact_chat_id, msg_c.as_ptr());
+    error!(context, 0, "{} ({})", msg, as_str(details));
     dc_contact_unref(contact);
 }
 
@@ -939,15 +939,15 @@ pub unsafe fn dc_handle_degrade_event(context: &Context, peerstate: &Peerstate)
                 &mut contact_chat_id,
                 0 as *mut libc::c_int,
             );
-            let c_addr_ptr = if let Some(ref addr) = peerstate.addr {
-                to_cstring(addr)
-            } else {
-                std::ptr::null_mut()
+            let peeraddr: &str = match peerstate.addr {
+                Some(ref addr) => &addr,
+                None => "",
             };
-            let msg = dc_stock_str_repl_string(context, 37, c_addr_ptr);
-            dc_add_device_msg(context, contact_chat_id, msg);
-            free(msg as *mut libc::c_void);
-            free(c_addr_ptr as *mut _);
+            let msg = CString::new(
+                context.stock_string_repl_str(StockMessage::ContactSetupChanged, peeraddr),
+            )
+            .unwrap();
+            dc_add_device_msg(context, contact_chat_id, msg.as_ptr());
             context.call_cb(
                 Event::CHAT_MODIFIED,
                 contact_chat_id as uintptr_t,