mavutil: add force-connect option

[peterbarker]

This will allow sim_vehicle.py to regain gdb_stopped.

It also allows you to start mavproxy without a named serial device attached, and then connect that device.

[khancyr]

@peterbarker Ok, I found one error, I try with a 3dr radio and now it don't crash if not plugged, but when I plug it does nothing.

[khancyr]

It appear that autoreconnect is try only once when the first write failed. mavproxy should keep trying to write until it got it working.

Otherwise, instead of using a fakeserial, we could have use https://pythonhosted.org/pyserial/shortintro.html#configuring-ports-later and try to open the port only if it appear is port list

[amilcarlucas]

Needs a rebase