p9fs: Use UNLINKAT instead of REMOVE to implement removals

REMOVE doesn't work properly in the face of hard links. Use UNLINKAT
instead, which is implemented by qemu and bhyve and lets the client
specify the name being removed.

PR: 282432
Reviewed by: dfr
Differential Revision: https://reviews.freebsd.org/D47438

(cherry picked from commit 1d99e8d9a37e1ba528628186df57b79fe74e196c)