Fix no_host_referral concatention in KDC

If no_host_referral is set in both [kdcdefaults] and the realm
subsection, we're supposed to concatenate their values.  But the logic
in handle_referral_params would overwrite the value with the
non-concatenated realm value.  Similar bugs of this nature were fixed
in 639c9d0f5a7c68dc98a2a452abc05ca32443cddf (r22037) but this one was
missed.

ticket: 7539 (new)
target_version: 1.11.1
tags: pullup
Clean up k5_locate_server error handling

profile_get_values() cannot return success with an empty list of
values, so don't bother counting them.  Return 0 from
locate_srv_conf_1 if no profile values exist and from
dns_locate_server if we decide not to make a SRV query.  Adjust
k5_locate_server to match the new helper behavior, and return
KRB5_REALM_UNKNOWN if neither profile nor DNS come up with any answers
(not KRB5_REALM_CANT_RESOLVE, which doesn't make sense now that we're
deferring KDC hostname resolution).
Add k5test support for KDC special environments

Accept environment parameters in the K5Realm start_kdc and
start_kadmind methods, so that krb5kdc or kadmind can be started or
restarted with a modified configuration.
Test KDC host-based referral support

Test the KDC host-based referral support in t_referral.py, using a new
harness to call krb5_get_credentials with a specified server name
type.  Also use this new harness for the #7483 regression test, to
avoid relying on an undocumented kvno extension.
Remove manual kdc_realm tests