Add extv3 X509 field support to --x509-username-field

Add extv3 X509 field support to --x509-username-field

This allows using other X509 certificate fields for the certificate
authentication.  To use altSubjectName, use
--x509-username-field ext:altSubjectName

This feature requires OpenVPN to be built with --enable-x509-alt-username

This patch is slightly modified, to honour --enable-x509-alt-username
compile time configuration.  Two #ifdef's are added.

Signed-off-by: Markus Koetter <koetter@rrzn-hiwi.uni-hannover.de>
Signed-off-by: David Sommerseth <davids@redhat.com>
Acked-by: David Sommerseth <davids@redhat.com>
Signed-off-by: David Sommerseth <davids@redhat.com>