This is achieved by reworking the logic to look at next move:
if we know the next move will set absolute E if needed, we don't need
to do it for this line. We can thus skip this G-code.

The print thread must always respawn the sender thread when ending (the change
introduced by 9c55957e was checking stop_send_thread, which is set to False by
_start_sender, so it never got it). Instead of adding more logic, let's just
always respawn the thread and kill it immediately.

By setting printing to false and not restarting the sender thread when the print_thread exits in the disconnect method this should allow printcore to disconnect even if the print thread is running.

In particular this makes it possible to ctrl+c out of prontserve durring an m109 where it previously errored out and hung with the following error:

Can't read from printer (disconnected?) (SerialException): device reports readiness to read but returned no data (device disconnected?)

Conflicts:
	printcore.py