Fix kdb5_ldap_util stashsrvpw password file logic

Fix kdb5_ldap_util stashsrvpw password file logic

kdb5_ldap_util stashsrvpw has several inconsistencies with the
password file determination in libkdb_ldap, and could try to fopen() a
NULL filename in some cases.  Factor out the determination of the
configured password file and make it consistent with libkdb_ldap.
DEF_SERVICE_PASSWD_FILE is no longer used after these changes, as it
is not respected by libkdb_ldap.

Reported by Will Fiveash.

ticket: 8295