wg-quick: linux: don't fail down when using systemd-resolved

systemd-resolved has a compatibility interface for use with resolvconf
scripts when resolvectl is called from a symlink from resolvconf.
However, when tearing down the interface, cmd_down calls del_if and then
unset_dns. In the case of systemd-resolved, deleting the interface also
removes the systemd-resolved entry and causes resolvconf -d to fail when
resolvconf really is a symlink to resolvectl. This causes `wg-quick
down` and 'wg-quick@.service' to exit with failure.

Instead we use the resolvconf '-f' flag to ignore non-existent
interfaces, supported by both openresolv and sd-resolved resolvconf.

Signed-off-by: Ronan Pigott <rpigott@berkeley.edu>
[zx2c4: moved -f argument to end to remain compatible with Debian's resolvconf]
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

src/wg-quick/linux.bash

@@ -155,7 +155,7 @@ set_dns() {
 
 unset_dns() {
 	[[ ${#DNS[@]} -gt 0 ]] || return 0
-	cmd resolvconf -d "$(resolvconf_iface_prefix)$INTERFACE"
+	cmd resolvconf -d "$(resolvconf_iface_prefix)$INTERFACE" -f
 }
 
 add_route() {