tun.c: don't attempt to delete DNS and WINS servers if they're not set

tun.c: don't attempt to delete DNS and WINS servers if they're not set

Commits

    1c4a47f7 ("wintun: set adapter properties via interactive service")
    18826de5 ("Set WINS servers via interactice service")

added functionality of add/remove DNS/WINS via interactive
service, which is used mostly by dco-win and wintun (tap-windows6
normally uses DHCP). There is a check in code - if DNS/WINS addresses
are not pushed, nothing is added.

However, due to bug we always attempted to remove DNS/WINS,
even if nothing was added. Removing WINS, for example, could take
up to 3 seconds.

This change fixes this by improving check "has DNS/WINS been pushed?".

While on it, convert do_XXX_service() functions to "void" from "bool",
since we never check their return values.

Change-Id: I21a36d24f8e213c780f55acbe3e4df555c93542a
Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20231220133637.60996-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg27843.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit c590868a721881dd21bfb77ecf846e6c8720e4ef)