Provide an option to specify interface node

This should be useful when you want to pre-create tun interface and give it a meaningful name at the same time, e.g. now it's possible to do the following on FreeBSD:

cloned_interfaces="tun101" create_args_tun101="name vpn-work"

openconnect --interface vpn-work --interface-node /dev/tun101 ...

Previously, different rc scripts were using hacks to rename the interface after the connection is established, this change should make those unneeded.

(Somewhat inspired by the 'dev' and 'dev-node' options in OpenVPN.)