[POSIX][Bug] syslog: Add support for %m modifier

The POSIX standard states that the `syslog()` function generates
the body from the message and arguments the same way as `printf()`,

> except that the additional conversion specification `%m` shall be
> recognized;

*https://pubs.opengroup.org/onlinepubs/009695399/functions/syslog.html*

What most of the implementations do is to leverage the processing to
`vsprintf` internals, to reduce code duplicity. This means the `%m`
modifier is present on almost all `printf` implementations. Take
the following code snippet as an example: https://onlinegdb.com/YdR9pU6KS.

Therefore, for `syslog` to support such a specification, the underlying
library shall be updated to support it too.

libs/libc/stdio/lib_libvsprintf.c

@@ -163,6 +163,10 @@ static int vsprintf_internal(FAR struct lib_outstream_s *stream,
   uint16_t flags;
   int width;
   int prec;
+
+  /* For the %m format we may need the current `errno' value */
+
+  int saved_errno = errno;
   union
   {
 #if defined (CONFIG_LIBC_LONG_LONG) || (ULONG_MAX > 4294967295UL)
@@ -911,6 +915,11 @@ flt_oper:
           size = 1;
           goto str_lpad;
 
+        case 'm': /* Print error message (%m) */
+          pnt = strerror(saved_errno);
+          size = strlen(pnt); /* Adjusting the size is not supported by %m */
+          goto str_lpad;
+
         case 's':
         case 'S':
 #ifdef CONFIG_LIBC_NUMBERED_ARGS