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/basic/cgroup-util.c

@@ -669,7 +669,7 @@ int cg_get_xattr_malloc(const char *controller, const char *path, const char *na
         if (r < 0)
                 return r;
 
-        r = getxattr_malloc(fs, name, ret, false);
+        r = lgetxattr_malloc(fs, name, ret);
         if (r < 0)
                 return r;