flock: user-configurable exit code

flock: user-configurable exit code

When locking the file fails with -n or -w option, caller has no way
to distinguish between the exit code 1 of the -c command,
and the exit code 1 of flock(1) caused by the conflicting lock.

Add a new -E <exitcode> (--conflict-exit-code) option to set
the exit code for the case of locking failure to any value.