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

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

Added in https://github.com/torvalds/linux/commit/6140be90ec70c39fa844741ca3cc807dd0866394

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:
https://github.com/torvalds/linux/commit/e896474fe4851ffc4dd860c92daa906783090346