Warn if push is used without --mode server/--server/--server-bridge

Warn if push is used without --mode server/--server/--server-bridge

This is not a supported configuration and will often work good enough
to get a connection working but will operate more in a weird pre P2P
negotiation compatibility way rather than actually negotiating
protocol features.

While at it, remove an unused macro (PUSH_DEFINED).

Change-Id: I82c7c61be07593ecd5bf2f854767dda74ab5170c
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1288
Message-Id: <20251023155614.20642-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg33856.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>