add netbsd case for tap
add a port option to specify type explicitly

this allows using the nic-side of tap device even if its name is
started with "tap".
eg.
	{port, 5, [{interface, "tap0"}, {type, tap}]},
	{port, 6, [{interface, "tap1"}, {type, eth}]}
Merge pull request #78 from yamt/tap

tap related changes