Clean up k5_locate_server error handling

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).