Use printf's %m format instead of strerror(errno) in more places

Most callers of strerror() are removed from the backend code.  The
remaining callers require special handling with a saved errno from a
previous system call.  The frontend code still needs strerror() where
error states need to be handled outside of fprintf.

Note that pg_regress is not changed to use %m as the TAP output may
clobber errno, since those functions call fprintf() and friends before
evaluating the format string.

Support for %m in src/port/snprintf.c has been added in d6c55de1f9,
hence all the stable branches currently supported include it.

Author: Dagfinn Ilmari Mannsåker
Discussion: https://postgr.es/m/87sf13jhuw.fsf@wibble.ilmari.org

src/backend/postmaster/syslogger.c

@@ -1173,7 +1173,7 @@ write_syslogger_file(const char *buffer, int count, int destination)
 	 * to our input pipe which would result in a different sort of looping.
 	 */
 	if (rc != count)
-		write_stderr("could not write to log file: %s\n", strerror(errno));
+		write_stderr("could not write to log file: %m\n");
 }
 
 #ifdef WIN32