Move protocol option negotiation from push_prepare to new function

Move protocol option negotiation from push_prepare to new function

This clean ups the code and removes the surprising side effects
of preparing a push reply to also select protocol options.

We also remember if we have seen a push request without async
push. This improves reaction time if deferred auth is involved
like managment interface deferred auth.  The other benefit is
removing a number of ifdefs.

NOTE: this patch breaks asynchronous authentication (via plugins
and possibly also via management interface).  The next commit will
fix this.  This is understood and hereby documented, but the two
individual commits are much cleaner without trying to fix it here
or squash both together.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20200709101603.11941-4-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20255.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>