Relative mode (M83) persists even when printing UltiGCode

[ErikDeBruijn]

Problem: Stripped filament at the feeder end, printer keeps failing on all prints that follow unless printer is reset.

Current behavior:
When you've used your own gcode which uses relative E coordinates (M83), for any print that follows, (even with UltiGCode) the printer will "assume" that these prints also have relative E codes. This means that the printer will rapidly spin up the feeder and keep spinning up until the filament is stripped.

Desired behavior:
Printing UltiGCode should yield predictable results, which don't depend on what you've printed before this UltiGCode. While printing from USB should obviously keep the state (e.g. relative versus absolute).

This can be replicated with a UM 2 with firmwares 14.09.0, 15.02.1 and 15.04.