After calling planner_abort_hard() no motion command can be
scheduled until we return to the main loop since the call can
potentially be scheduled inside a nested process_command call.
Despite previous fixes, bugs keep creeping in due to nested calls not
being obvious to detect at all.
Stop allowing motion _completely_ for the entire processing loop by
default. That is, instead of aborting the current plan_buffer_line call,
abort the entire command until we can actually schedule motion safely
again.
This benefits handling of pretty much all g/m-codes, since this flag
(now "planner_aborted" for clarity) becomes a general "command aborted"
call.
This also now ensures that the flag prevents _any_ new block (including
blocks partially planned while servicing an interrupt) are scheduled
after planner_abort_hard is called.
There are only two exceptions where it's safe to resume in this context:
- Within uvlo_, where we never return to the main processing loop
- When we're intentionally scheduling a new process_command loop for a
MK3 filament recheck (which is *bad*)
Handle those two cases as exceptions.
