dco: print version to log if available

dco: print version to log if available

In order to provide better support in case of troubleshooting issues,
it's important to know what exact DCO version is loaded on the user
system.

Therefore print the DCO version during bootup.

For Windows and FreeBSD we currently implement a placeholder printing 'v0'.
This should be improved with a follow-up patch.

For Linux we directly fetch the module version from /sys and print
something like:

	DCO version: 0.1.20230206-15-g580608ec7c59

Change-Id: Ie1f6fa5d12a473d353d84fd119c2430b638e8bcd
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20230309131419.29157-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26370.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>