Macro conflicts when using multiple motion systems

[dc42]

When the firmware supports multiple motion systems, macros invoked by a job file are executed by both motion systems. This often causes macros to fail if they haven't been designed with this in mind. For example, the conditions within if, elif and while commands may rely on global variables that haven't been created yet. See https://forum.duet3d.com/topic/35023/mutiple-motion-systems-macro-conflicts-in-3-5.

[dc42]

Fixes proposed:

1. Change behaviour so that the 'abort' command is only executed by the input stream that executes commands for the active motion system (as is already the case for global, set global and echo commands).
2. Add a new M606 command to request execution of multiple motion systems by separate file readers explicitly. Until this command is seen, use the File stream to execute all commands.

[dc42]

Implemented fix 1 above in 3.5-dev branch. Created new branch 3.5-mms-changes and implemented fix 2 in it. Binaries at https://www.dropbox.com/scl/fo/3y4agkmfzfmqcifuecqo3/h?rlkey=j0kibs1tubm5dfj7o2vz1vbzj&dl=0 have been made available to users for testing.

[dc42]

3.5-mms-changes now merged into 3.5-dev.