Support complex LDAP organizational structures

The current LDAP authentication module in Guacamole is very basic. It takes the username and adds the LDAP base DN. This is not sufficient for a directory where users are scattered across multiple distinct OUs under the base DN.

The LDAP module should instead:

1. Perform a search for the parameter which identifies the user (uid/samaccountname) under the ENTIRE subtree from the base DN
2. Use just the uid/samaccountname to identify the user