Initial implementation of Kerberos password verification for

simple bind via:
	{KERBEROS}principal
Code is disabled by default (for security reasons).  Use
--enable-kpasswd to enable.  Behind SLAPD_KPASSWD.
Reworked Kerberos detection and split out KBIND as independent
feature (--disable-kbind) (LDAP_API_FEATURE_X_OPENLDAP_V2_KBIND).
KBIND depends upon detection of KRB4 (or KRB425) support.  Detection,
building with eBones (as distributed with FreeBSD 3.4) okay, but
wasn't able to test as I don't have a K4 KDC handy.
--with-kerberos has a number of detection options... most likely
don't work properly.