Changed default tls_opt_require_cert value to LDAP_OPT_X_TLS_DEMAND; force

a fatal error when TLS server cert verification fails.

Changed ldap_pvt_tls_check_hostname to return LDAP_SUCCESS when no cert is
found: this can now only occur if tls_opt_require_cert was explicitly set
to NEVER or ALLOW.

In tls_verify_cb, added a text translation of the verification error code
to the debug message.