Allow M98 P1 L0 for subs

[cncdrive]

Hi Terry,

I beleive L0 should call the subroutine zero times, because L defines how many times the sub is called, so with L being zero value the sub is not called.
It works like this in the UCCNC and the same way in Mach3.
And if the L is not defined then it is interpreted as if L was 1, so the sub runs once, this also works the same in UCCNC and Mach3.