Remove the 'state' variable of per-channel data, use active flag exclusively to track
open/close state.

Add misdn_activate_channel()/misdn_deactivate_channel() helper functions, rename old
one to _misdn_toggle_channel() (internal).

Add _nowait variant of channel de-/activation function, that just sends the mISDN request message.
Signed-off-by: Stefan Knoblich <stkn@openisdn.net>