Simplify LDAP module by relying on OpenLDAP 2.1

Simplify LDAP module by relying on OpenLDAP 2.1

Solaris 11 provides an OpenLDAP library (which we don't auto-detect,
but should) in addition to the old Mozilla LDAP library; this will
become the default LDAP library in the next release.  As there is no
longer a need to build against the Mozilla LDAP library, and as we
have unwittingly relied on some OpenLDAP-specific features since 1.13,
remove the compatibility code for the Mozilla LDAP library and just
require OpenLDAP 2.1 (which added ldap_str2dn).