Add LDAP constants

[haszi]

I did not write any descriptions for the peer certificate checking and CRL evaluation strategy constants as I could not find any references discussing these in detail.

Peer certificate checking strategies: LDAP_OPT_X_TLS_ALLOW LDAP_OPT_X_TLS_DEMAND LDAP_OPT_X_TLS_HARD LDAP_OPT_X_TLS_NEVER LDAP_OPT_X_TLS_TRY

CRL evaluation strategies: LDAP_OPT_X_TLS_CRL_ALL LDAP_OPT_X_TLS_CRL_NONE LDAP_OPT_X_TLS_CRL_PEER