[krbdev.mit.edu #2611] automatically getting initial tickets using keytab

[Ken Raeburn via RT]

We've got the ability to automatically prompt for a password when
tickets have expired on Mac OS X, and the Windows code is doing some
magic dealing with refreshing tickets too.

It would be handy to have a way to say, here's my keytab file, go get
new tickets any time they're needed.  Then a long- or
frequently-running service or batch job or whatever with a keytab
doesn't have to explicitly re-run kinit every N hours.

Means of specifying this behavior TBD...

(Suggested at cartel meeting.)

Ken