lockf: add a -p mode to write the child's pid

[kevans]

Summary

If we're going to hold the lock, it can be useful to scribble down the
pid that we spawned off to quickly associate the lock back to the
process that's keeping it open.

[allanjude]

Very useful functionality

[des]

usr.bin/lockf/lockf.c

264

Why not just (void)dprintf(lockfd, "%d\n", child)?

[kevans]

usr.bin/lockf/lockf.c

264

I didn't think I really wanted the extra complexity, but looking at the implementation it doesn't seem to be as bad as I was afraid it might be

[kevans]

implify with dprintf

[des]

usr.bin/lockf/lockf.c

94

I would prefer if the flags were bools but that's cosmetic.

[kevans]

usr.bin/lockf/lockf.c

94

Yeah, I was eyeballing both this and perhaps the 'kill(getpid(), ....)` pattern in the signal handler for a cosmetic pass.

[kevans]

usr.bin/lockf/lockf.c

124

Adding some flags |= O_TRUNC here as well, in case we're rolling with -k.