add pidfile_signal and use it in mount

[mjg]

Summary

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. this induces contention against anything doing an exec as it tries to look up /var/run/ld-elf.so.hints

commit 963072c7bae56d6f4d53f52daab6ca23eecff487 (HEAD -> pidfile)
Author: Mateusz Guzik <[e-mail]>
Date:   Fri Mar 11 11:02:28 2022 +0000

    mount: use pidfile_signal

commit 2796d6df18f81e69ee22d43528e275c9cfe3be47
Author: Mateusz Guzik <[e-mail]>
Date:   Fri Mar 11 11:01:50 2022 +0000

    pidfile: add pidfile_signal

Test Plan

started mountd and verified it gets signalled, if running

[rew]

/usr/src/lib/libutil/pidfile.c:369:2: error: implicit declaration of function 'kill' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
        kill(pid, sig);

Missing <signal.h>, <sys/types.h>?

[mjg]

• add missing headers