Test that PAC is the first authdata element

In the test KDB module, set the PAC as the first authdata element.  In
adata.c, add PAC service verification and verify that a PAC does not
appear in authdata elements after the first.

[ghudson@mit.edu: minor style changes; edited commit message]

(cherry picked from commit d40d7c8ee8d7fb547e45c545365b21a818050130)

https://github.com/krb5/krb5/commit/011736101525c8f1bd6c2219462cc59c925da77e
Author: Isaac Boukris <iboukris@gmail.com>
Committer: Greg Hudson <ghudson@mit.edu>
Commit: 011736101525c8f1bd6c2219462cc59c925da77e
Branch: krb5-1.18
 src/plugins/kdb/test/kdb_test.c |    7 +++--
 src/tests/adata.c               |   54 +++++++++++++++++++++++++++++++++-----
 2 files changed, 51 insertions(+), 10 deletions(-)