Fix lockfile handling
I've ran bup-cron with sudo
and then tried to run it as a normal user, too:
configured stdout level 10
bup-cron 1.3 starting
locking pidfile /mnt/backup/localhost.bup-cron/.bup-cron.pid
can't deliver signal to 30333
removed staled lockfile /mnt/backup/localhost.bup-cron/.bup-cron.pid
...
Additionally, init and clear were not covered/protected by the pidfile.