tun.c: refactor open_tun() implementation

tun.c: refactor open_tun() implementation

This makes Windows's tun_open() method easier to read
by factoring out blocks of code, which perform certain task,
into separate functions. This also minimizes inflation of

  if (!tt->wintun)  { }

blocks.

While patch looks big and scary, there are no functional changes
at all, just tossing code around.

Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Simon Rozman <simon@rozman.si>
Message-Id: <20191113104216.1545-1-lstipakov@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg19137.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>