tun.c: make Windows device lookup functions more general

tun.c: make Windows device lookup functions more general

Since the introduction of Wintun, not all network devices in Windows are
TAP-Windows6. Rather than returning a simple true/false answer, a couple
of functions were reworked to return a corresponding struct tap_reg *
or NULL instead.

As it would make the code `tr = is_tap_win(...)` a bit awkward those
functions (both static) were renamed to better reflect their nature.

Signed-off-by: Simon Rozman <simon@rozman.si>
Acked-by: Lev Stipakov <lstipakov@gmail.com>
Message-Id: <20191220161117.1434-1-simon@rozman.si>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg19280.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>