build: Remove --disable-server from ./configure

build: Remove --disable-server from ./configure

After some discussion among the core community developers [1,2], it was
decided to remove the possibility to build openvpn as a pure client.
This was alterted on the mailing list [3] that it was scheduled for
removal unless anyone had strong arguments why it was needed.

The general consensus was that we had not received any strong arguments
to keep this possibility after approximately 5 months, so it was fine to
remove this ./configure option.

By removing this, we remove quite some entangled sections of #ifdef
scattered all over the code base, making it more readable.

One note:
Inside the  options_postprocess_mutate_invariant() function,
the #ifdef P2MP_SERVER and #ifdef _WIN32 blocks where slightly
reworked to make the _WIN32 block more continous and avoiding having an
empty if(options->mode == MODE_SERVER) block.

Signed-off-by: David Sommerseth <davids@openvpn.net>
[1]
https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18830.h
tml
[2]
https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg19505.h
tml
[3]
https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18829.h
tml
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20200227205443.27562-1-davids@openvpn.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg19506.html

Signed-off-by: Gert Doering <gert@greenie.muc.de>