debathena-msmtp should sanity-check principal before setting from

[jdreed]

While #806 is already being addressed, it's possible that something with host tickets could want to send mail. Setting the from to be host@… or daemon@… (as a result of stripping the instance off the principal host/foo.mit.edu) is not terrible, but also not great.

Note that sanity checking this is harder than you think. We can't just look them up in Hesiod, because daemon has a passwd entry. It's possible that the right thing to do here is to see if the instance looks like a hostname, and if so, set the from to root@hostname. Or whine at the user on STDERR.