always exclude TUNDEV when finding/setting gateway route

always exclude TUNDEV when finding/setting gateway route

The previous commit didn't fix #20, because 'ip route get' doesn't preserve the onlink flag.
(See https://gitlab.com/openconnect/vpnc-scripts/-/issues/20#note_542783676)

We should just use the 'ip route show' version across-the-board; it's more complex, but appears
to be quite robus. If we have to use it to get the gateway route correctly, then we simply
remove the redundancy in the code. *BSD code ('route'-based) can be similarly simplified.

Signed-off-by: Daniel Lenski <dlenski@gmail.com>