Fix combination of --dev tap and --topology subnet across multiple platforms.

Fix combination of --dev tap and --topology subnet across multiple platforms.

--topology should have no effect in tap mode (tap is always "subnet"),
but due to the way options are checked, setting "topology subnet" caught
an improper branch on all non-linux and non-win32 platforms.

Easily tested by adding "--topology subnet" to a "--dev tap" t_client
test.

Tested, verified, and fixed on FreeBSD 10.4, NetBSD 7.0.1, OpenBSD 6.0,
and OpenSolaris 10.  Compile-tested on MacOS X.

Trac: #1085

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20180819200703.20362-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17414.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>