lockf: add a -T option to terminate the child upon early abort

[kevans]

Summary

This is useful to avoid having the command running twice in the face of
the admin terminating the process. Notably, if the -p option is not in
use (or can't be used, e.g., because we can't open the file for writing)
then this provides a nice alternative where one simply needs to send a
SIGTERM to the lockf(1) process associated with the lock file to clean
it all up.

[des]

usr.bin/lockf/lockf.c

68

Should you also convert fdlock, keep, silent, and writepid to bool?

[kevans]

usr.bin/lockf/lockf.c

68

The cosmetic changes are later in the stack, though I did miss fdlock: https://reviews.freebsd.org/D51027