wg-quick: linux: deal with resolvconf migration more gracefully

wg-quick: linux: deal with resolvconf migration more gracefully

When Ubuntu systems upgrade from Debian's atrocious resolvconf to the
systemd-resolve symlink, they sometimes leave around
/etc/resolvconf/interface-order, which then winds up breaking the
detection and passing the bogus 'tun.' prefix to systemd's resolvconf,
resulting in failure. Work around this by only doing the 'tun.' prefix
hack if resolvconf isn't a symlink. This is ugly but so it goes.

Reported-by: Andrei Borzenkov <arvidjaar@gmail.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>