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.

this introduces a change in behavior: previously, bup clear wouldn't be ran right after an init, but now it would. is this intentional? if so it should be documented in the commitlog. i'm not sure how i stand on this.