add click inputswitch

InputSwitch is like Switch, except it selects an input port rather than
an output port. Unlike PullSwitch, InputSwitch uses push processing.

For example, for a wireless roaming client you can now say:
    ap0, ap1, ap2
    => cli_in_sw :: InputSwitch(0)
    -> cli
    -> cli_out_sw :: Switch(0)
    => ap0, ap1, ap2;

and then:
    write cli_in_sw.switch 1,
    write cli_out_sw.switch 1,
to make it roam to ap1.

Reviewed-by: cliff@meraki.com
Signed-off-by: Patrick Verkaik <patrick@meraki.net>