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.

won't this try to create the pid file inside the potentially not-yet existing `BUP_DIR`?