Choose a different field in X509 to be username

Choose a different field in X509 to be username

For my company, we use a PKI (linked to a LDAP) with OpenVPN. We can't use "CN" to be
username (few people can have the same "CN"). In our case, we only use the UID.

With my patch, you can choose another field to be username with a new option called
--x509-username-field, the default value is "CN".

Signed-off-by: Emilien Mantel <emilien.mantel@businessdecision.com>
Acked-by: David Sommerseth <dazo@users.sourceforge.net>
Signed-off-by: David Sommerseth <dazo@users.sourceforge.net>