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/tools/ifaddrs/test_ifaddrs.c

@@ -66,6 +66,6 @@ main(int argc, char *argv[])
 #endif
 
 	if (pg_foreach_ifaddr(callback, NULL) < 0)
-		fprintf(stderr, "pg_foreach_ifaddr failed: %s\n", strerror(errno));
+		fprintf(stderr, "pg_foreach_ifaddr failed: %m\n");
 	return 0;
 }