Add function for common env setting of verify user/pass calls

Add function for common env setting of verify user/pass calls

This removes the code duplication in verify_user_pass_script,
verify_user_pass_plugin and verify_user_pass_management.

This also fixes a bug that username is not set if auth-gen-token is
used without the external-auth flag as without calling any external auth
method, the environment would not be setup for connect-client calls.

This patch also removes an indentation level in most of touched functions
so diffing without whitespaces is recommended for review.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20201005111614.29325-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21174.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit a4eeef17b20541a7afde0f1cbeae4a4e2b0c455a)