add jdk.jndi.ldap.mechsAllowedToSendCredentials property

[klasen]

Added Property to Control LDAP Authentication Mechanisms Allowed to Authenticate Over Clear Connections

A new environment property, jdk.jndi.ldap.mechsAllowedToSendCredentials, has been added to control which LDAP authentication mechanisms are allowed to send credentials over clear LDAP connections - a connection not secured with TLS. An encrypted LDAP connection is a connection opened by using ldaps scheme, or a connection opened by using ldap scheme and then upgraded to TLS with a STARTTLS extended operation. JDK-8237990 (not public)