build: enable missing OpenSSF-recommended warnings, with fixes

Ref:
https://best.openssf.org/Compiler-Hardening-Guides/Compiler-Options-Hardening-Guide-for-C-and-C++.html
(2023-11-29)

Enable new warnings:

- replace `-Wno-sign-conversion` with `-Wsign-conversion`.

  Fix them in example, tests and wincng. There remain about 360 of these
  warnings in `src`. Add a TODO item for those and disable `-Werror` for
  this particular warning.

- enable `-Wformat=2` for clang (in both cmake and autotools).

- enable `__attribute__((format))` for `_libssh2_debug()`,
  `_libssh2_snprintf()` and in tests for `run_command()`.

  `LIBSSH2_PRINTF()` copied from `CURL_TEMP_PRINTF()` in curl.

- enable `-Wimplicit-fallthrough`.

- enable `-Wtrampolines`.

Fix them:

- src: replace obsolete fall-through-comments with
  `__attribute__((fallthrough))`.

- wincng: fix `-Wsign-conversion` warnings.

- tests: fix `-Wsign-conversion` warnings.

- example: fix `-Wsign-conversion` warnings.

- src: fix `-Wformat` issues in trace calls.

  Also, where necessary fix `int` and `unsigned char` casts to
  `unsigned int` and adjust printf format strings. These were not
  causing compiler warnings.

  Cast large types to `long` to avoid dealing with printf masks for
  `size_t` and other C99 types. Existing code often used `int` for this.
  I'll update them to `long` in an upcoming commit.

- tests: fix `-Wformat` warning.

- silence `-Wformat-nonliteral` warnings.

- mbedtls: silence `-Wsign-conversion`/`-Warith-conversion`
  in external header.

Closes #1257

example/sftp_write_sliding.c

@@ -244,7 +244,7 @@ int main(int argc, char *argv[])
                 break;
         }
         memuse += nread;
-        total += nread;
+        total += (libssh2_struct_stat_size)nread;
 
         /* write data in a loop until we block */
         while((nwritten = libssh2_sftp_write(sftp_handle, mem, memuse)) ==
@@ -254,10 +254,10 @@ int main(int argc, char *argv[])
         if(nwritten < 0)
             break;
 
-        if(memuse - nwritten) {
+        if(memuse - (size_t)nwritten) {
             /* make room for more data at the end of the buffer */
-            memmove(&mem[0], &mem[nwritten], memuse - nwritten);
-            memuse -= nwritten;
+            memmove(&mem[0], &mem[nwritten], memuse - (size_t)nwritten);
+            memuse -= (size_t)nwritten;
         }
         else
             /* 'mem' was consumed fully */