mount: use pidfile_signal

This fixes a performance problem where poudriere -j 104 keeps remounting
filesystems, which induce wanting to signal mountd, if running. The
current code tries to do it by creating the pidfile in /var/run and
unlinking it if the operation succeeds, inducing contention against
anything doing an exec as it tries to look up /var/run/ld-elf.so.hints

Differential Revision: https://reviews.freebsd.org/D34681