Apply permitted_enctypes to KDC request enctypes

Apply permitted_enctypes to KDC request enctypes

permitted_enctypes was initially intended only to restrict the
processing of AP requests (and was later applied to KDB key data
searches so that the KDC wouldn't issue a ticket it would refuse to
accept).  Because the documentation was never clear about its scope,
many configurations assume that permitted_enctypes also applies to
clients.

In light of the existing configurations, take the simple way out and
use permitted_enctypes as the default for default_tkt_enctypes and
default_tgs_enctypes.  Update the documentation, add a test to
explicitly check the new behavior, and remove now-unnecessary
configuration from the test suite.

[ghudson@mit.edu: unrolled helper function; edited documentation and
commit message; simplified test case]

ticket: 8869 (new)
tags: pullup
target_version: 1.18