Autonomous backup daemon.

Backd is a daemon, looking at backup directive and doing
everything as an autonomous process, I try to have it as
simple and forgiving as possible.

The idea behind backd, is to just receive mail asking
to insert a tape with a specific ID in the tape device,
backd will do the backup and advice sysadmin when the
scheduled backup is completed, requesting then the next
tape for the next schedled backup (and so one....)