Make the --x509-username-field feature an opt-in feature

Make the --x509-username-field feature an opt-in feature

After some discussion [1] regarding an extension of this feature,
James Yonan wanted this extension to be an opt-in feature.  However,
as it does not make sense to opt-in on a extension of a feature which
was discussed, this patch makes the base feature an opt-in instead.

The base feature comes from commit 2e8337de248ef0b5b48cbb2964 (beta2.2)
and commit 935c62be9c0c8a256112 (feat_misc).

[1] http://thread.gmane.org/gmane.network.openvpn.devel/4266

Signed-off-by: David Sommerseth <davids@redhat.com>
Acked-by: James Yonan <james@openvpn.net>
(cherry picked from commit 024972e2ced84c6e5cabc43620ab510e5693d1d4)