wg-quick: account for specified fwmark in auto routing mode

wg-quick: account for specified fwmark in auto routing mode

If we're doing automatic routing with default routes, but the config has
also specified an explicit fwmark, then use that explicit fwmark, even
if it's conflicting, since the administrator has explicitly opted into
using it. Also, when shutting down the interface, we only now remove the
fancy rules if we're in automatic routing mode with default routes.

Suggested-by: Luis Ressel <aranea@aixah.de>
Reported-by: Saeid Akbari <saeidscorp@yahoo.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>