Enable KDC TCP listener by default

Make the KDC default to listening on TCP.

ticket: 6731
target_version: 1.13
tags: pullup
Document KDC TCP listener change

Update documentation to reflect the change in the default KDC TCP
listener behavior, new in 1.13.

ticket: 6731
target_version: 1.13
tags: pullup
Fix ksu crash in cases where it obtains the TGT

In order to allow ksu to use any locally-present service key for
verifying creds, the previous change to ksu switched from using a
retrieved or obtained TGT to fetch creds for the local "host" service,
and then passing those creds to krb5_verify_init_creds(), to passing the
retrieved TGT directly to krb5_verify_init_creds().

It did not take care to retrieve the TGT from the temporary ccache if it
had obtained them, and in those cases it would attempt to verify NULL
creds.

Modify the krb5_get_tkt_via_passwd() function to call
krb5_get_init_creds_password(), to pass back the freshly-obtained creds,
to take a "krb5_get_init_creds_opt" pointer instead of a locally-defined
options structure, and rename it to ksu_get_tgt_via_passwd().