fix typo
add very simple PID file handling to prevent multiple concurrent jobs

Oops, the LOCK file handling was only for preventing running after error --
it did nothing to prevent multiple backup jobs from running at the same time.
Normally not a problem, but a slow resilver on the destination could cause
overlapping jobs.

This is just about the bare minimum, and can/should be improved.
zfssnap user doesn't have write access to /var/run, so use /var/tmp

Perhaps this could be allowed with an ACL or RBAC, or use some std.
SMF PID file handling, but for now just use a dir we're guaranteed
to be able to write to.