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/port/path.c

@@ -772,8 +772,7 @@ make_absolute_path(const char *path)
 #ifndef FRONTEND
 				elog(ERROR, "could not get current working directory: %m");
 #else
-				fprintf(stderr, _("could not get current working directory: %s\n"),
-						strerror(errno));
+				fprintf(stderr, _("could not get current working directory: %m\n"));
 				return NULL;
 #endif
 			}