[krbdev.mit.edu #7190] Try harder to make keytab-based AS requests work

[Greg Hudson via RT]

Nico also suggests making gic_keytab use encrypted timestamp preauth 
(http://mailman.mit.edu/pipermail/krbdev/2012-July/010999.html), and 
making the KDC use the encrypted timestamp key as the reply key.  These 
are both reasonable ideas, but the former may have some edge cases.