[LinuxCNC/linuxcnc PR#186] Preview “ungrounded” programs better

This pull request tackles a different part of issue #185: better preview of “ungrounded” programs.

This is done by having the preview-interpreter execute a bunch of additional preparatory gcodes, to
– set the initial position
– load the tool, if any
– set the tool length offsets, if any
– apply all modal G- and M-codes based on the stat buffer

As a result, my test “ungrounded” program behaves a lot better as I do things like jog around, MDI to load/unload a tool, etc.

;ungrounded.ngc
f100
g42
G1 X1
G1 X5
G1 Y8
M2