Add user-visible support for trunking.

Added support for Cisco switches to enable and disable trunking for a
port.

Added two new command-line options:
-N <port> <vlans>
	Enable trunking on a port, allowing the given list of VLANs to
	cross the trunk

-U <port>:
	Disable trunking for the port

This functionality is needed for Mike's firewall support.