tuncer: support flow control

Add basic flow control using setopt/2 (note: it is setopt (singular) not
setopts like in inet). The second argument to setopt/2 is a tuple.
Currently supported:

* {active, false} : disable the erlang port

* {active, true} : select on the tuntap fd using an erlang port

This allows the caller to temporarily stop receiving messages without
closing the tuntap device.
controlling_process: preserve the message order
Update README
send: support {active,false} mode

Do the equivalent of write/2 if the tuntap interface is set for {active,
false} mode instead of crashing.