docs: mbox -> mailbox

todo: explain somewhere what 'mailbox' means?

todo: rename or change --show-mbox-path option
docs: change from md5 to mentioning `mailbox_path_hash`

have only searched for md5, though, not read through everything,
really done?
done: solveable using `mailbox_path_hash` config

although todo, provide something out of the box?
use usync instead of rsync for keeping times

usync from github/pflanze/chj-bin