Mk/**ldap.mk: Convert USE_LDAP to USES=ldap

Convert the USE_LDAP=yes to USES=ldap and adds the following features:

• Adds the argument USES=ldap:server to add openldap2{4|5|6}-server as RUN_DEPENDS
• Adds the argument USES=ldap<version> and replaces WANT_OPENLDAP_VER
• Adds OPENLDAP versions in bsd.default-versions.mk
• Adds USE_OPENLDAP/WANT_OPENLDAP_VER in Mk/bsd.sanity.mk
• Changes consumers to use the features

Reviewed by: delphij
Approved by: portmgr
Differential Revision: https://reviews.freebsd.org/D38233