Add tldap_context_[gs]etattr

Add tldap_context_[gs]etattr

This adds the ability to attach extended information to a tldap_context. This
will become useful once we start to do automatic reconnects for example, a
callback function might want attach a pointer to credentials so that it can
rebind.

The initial user of this will be a cached rootdse, so that things like the
ability to do paged searches can be cached.