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/cgroup-show.c

@@ -127,7 +127,7 @@ static int show_cgroup_name(
         bool delegate = false;
         int r;
 
-        r = getxattr_malloc(path, "trusted.delegate", &b, false);
+        r = lgetxattr_malloc(path, "trusted.delegate", &b);
         if (r < 0) {
                 if (r != -ENODATA)
                         log_debug_errno(r, "Failed to read trusted.delegate extended attribute: %m");