xattr-util: merge various getxattr()/listxattr() helpers into getxattr_at_malloc() + listxattr_at_malloc()

Unfortunately fgetxattr() and flistxattr() don't work via O_PATH fds.
Let's thus add fallbacks to go via /proc/self/fd/ in these cases.

Also, let's merge all the various flavours we have here into singular
implementations that can do everything we need:

1. malloc() loop handling
2. by fd, by path, or combination (i.e. a proper openat() like API)
3. work on O_PATH

src/shared/discover-image.c

@@ -334,7 +334,7 @@ static int image_make(
                 if (!ret)
                         return 0;
 
-                (void) fd_getcrtime_at(dfd, filename, &crtime, 0);
+                (void) fd_getcrtime_at(dfd, filename, AT_SYMLINK_FOLLOW, &crtime);
 
                 if (!pretty) {
                         r = extract_pretty(filename, ".raw", &pretty_buffer);