Tag my_vsnprintf.c with ATTRIBUTE_FORMAT

[Breaking]
Good news:
GCC now checks your `my_snprintf` (direct) calls (`-Wformat` was on);
Bad news:
The build process no longer lets your incorrect
formats/arguments sneak past (`-Werror` was also on).

As such, this commit also migrates all direct `my_snprintf` calls from
the old specifiers to MDEV-21978’s new `-Wformat`-compatible suffixes.
The next commits will cover non-direct calls to `my_snprintf`.
(I call them “`my_snprintf` descendants”.)

This commit does not update the ABI records because
there’re more ABI “changes” to come – half a dozen
`include/mysql/plugin_*.h.pp`s are now missing the new `__attribute__`s.

client/mysql_upgrade.c

@@ -556,7 +556,7 @@ static void find_tool(char *tool_executable_name, const char *tool_name,
 
     len= (int)(last_fn_libchar - self_name);
 
-    my_snprintf(tool_executable_name, FN_REFLEN, "%.*b%c%s",
+    my_snprintf(tool_executable_name, FN_REFLEN, "%.*sB%c%s",
                 len, self_name, FN_LIBCHAR, tool_name);
   }