[LinuxCNC/linuxcnc PR#67] Seb/2.6/task abort fix

This fixes #49 and the Task abort/statbuffer issue that Zultron reported.

I wish it wasn’t so invasive, so late in the 2.6 and 2.7 stable branches, but it fixes real bugs with as little impact as I can manage.

This branch changes behavior in a couple of ways:
– Abort now cleans up Interp and Canon just about as much as a real M2 or M30 would.
– Task startup results in a bunch more Canon & Motion commands being processed. These are the same commands it’s always generated (just about), but now it doesn’t discard them before processing them.
– The [RS274NGC]RS274NGCSTARTUPGCODE is now executed, before any UI commands are accepted by Task.