[lmilz]

Fixes #694

Adds a first implemention of flock with the core functionality from flock(1):

• the three forms: flock FILE CMD [ARGS…], flock FILE -c CMD and flock FD
• -s/-x/-e/-u (the last one wins, as in util-linux), -n/--nb, -w, -E, -c
• lock files are created if needed; directories and read-only files are opened read-only
• exit statuses follow util-linux: the -E code on conflict or timeout, the child's status (128+signal if it was killed), and the sysexits codes 64/65/66/69/71 for errors, with the same messages

Differences from util-linux:

• usage errors exit with 1 (clap) instead of 64
• -w polls with LOCK_NB every 5 ms instead of using a timer

Not implemented yet (follow-up PRs): -o, -F, --verbose, --fcntl, --start, --length.