Use non-blocking socket reads for rotctl track responses

[bjorgan]

Ref. #108: Flyby singletrack UI can
become unresponsive during tracking since we wait for confirmation of
the previous response before we try to set a new position.

These changes modify the socket read to be non-blocking, so that we can
return back to the UI loop when the full response is not available yet,
and still retain the mechanism with the previous response being required
before we are allowed to set a new position.

This does not fully solve #108 since
we will have an issue with the hamlib status windows, but I'll continue
on that in a different branch and merge this already now so that the
user experience can be improved.