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

include/libssh2.h

@@ -279,6 +279,7 @@ typedef struct _LIBSSH2_SK_SIG_INFO {
              const unsigned char *data, size_t data_len, void **abstract)
 
 /* 'keyboard-interactive' authentication callback */
+/* FIXME: name_len, instruction_len -> size_t, num_prompts -> unsigned int? */
 #define LIBSSH2_USERAUTH_KBDINT_RESPONSE_FUNC(name_) \
     void name_(const char *name, int name_len, const char *instruction, \
                int instruction_len, int num_prompts, \