lockf: minor cosmetic cleanups, no functional change

Switch various flags from int -> bool.

kill(getpid(), foo) and raise(foo) are equivalent in this context, so
switch to the latter as a minor preference for readability.

Use proper signal fences instead of volatile for our SIGALRM handler.

Reviewed by: kib (earlier version), des
Differential Revision: https://reviews.freebsd.org/D51027