Implement %m in src/port/snprintf.c, and teach elog.c to rely on that.

I started out with the idea that we needed to detect use of %m format specs
in contexts other than elog/ereport calls, because we couldn't rely on that
working in *printf calls.  But a better answer is to fix things so that it
does work.  Now that we're using snprintf.c all the time, we can implement
%m in that and we've fixed the problem.

This requires also adjusting our various printf-wrapping functions so that
they ensure "errno" is preserved when they call snprintf.c.

Remove elog.c's handmade implementation of %m, and let it rely on
snprintf to support the feature.  That should provide some performance
gain, though I've not attempted to measure it.

There are a lot of places where we could now simplify 'printf("%s",
strerror(errno))' into 'printf("%m")', but I'm not in any big hurry
to make that happen.

Patch by me, reviewed by Michael Paquier

Discussion: https://postgr.es/m/2975.1526862605@sss.pgh.pa.us

src/common/psprintf.c

@@ -45,6 +45,7 @@
 char *
 psprintf(const char *fmt,...)
 {
+	int			save_errno = errno;
 	size_t		len = 128;		/* initial assumption about buffer size */
 
 	for (;;)
@@ -60,6 +61,7 @@ psprintf(const char *fmt,...)
 		result = (char *) palloc(len);
 
 		/* Try to format the data. */
+		errno = save_errno;
 		va_start(args, fmt);
 		newlen = pvsnprintf(result, len, fmt, args);
 		va_end(args);
@@ -89,6 +91,9 @@ psprintf(const char *fmt,...)
  * Other error cases do not return, but exit via elog(ERROR) or exit().
  * Hence, this shouldn't be used inside libpq.
  *
+ * Caution: callers must be sure to preserve their entry-time errno
+ * when looping, in case the fmt contains "%m".
+ *
  * Note that the semantics of the return value are not exactly C99's.
  * First, we don't promise that the estimated buffer size is exactly right;
  * callers must be prepared to loop multiple times to get the right size.