xattr-util: try new *xattrat() family syscalls first

Added in 6140be90ec

However, when O_PATH fds are encountered we'd have to go by
/proc/self/fd/ still, since the kernel people are reluctant
to make the new syscalls work with them
(https://lore.kernel.org/linux-fsdevel/20250206-steril-raumplanung-733224062432@brauner/)
Hence getxattrat() and listxattrat() are not employed.

While at it, remove the discrepancy between path being NULL
and empty - I don't grok the "security issue" claimed earlier,
but nowadays even the kernel treats the two as identical:
e896474fe4

README

@@ -67,6 +67,7 @@ REQUIREMENTS:
                                and MOVE_MOUNT_BENEATH
                      ≥ 6.6 for quota support on tmpfs
                      ≥ 6.9 for pidfs
+                     ≥ 6.13 for PIDFD_GET_INFO and {set,remove}xattrat()
 
         ✅ systemd utilizes several new kernel APIs, but will fall back gracefully
            when unavailable.