Functionally, the only difference between the system definitions is that
GAMEL lets you specify an additional internal layout and SYST lets you
specify a compatible system. COMP and CONS are just aliases for SYST -
the aliases can be phased out.
Removed arcade.flt and mess.flt altogether - opinion seems to be split
between misinterpreting them as fully supported and considering them
unnecessary. They were marginally useful as a performance test for
makedep.py, but that isn't important. We still have nl.flt as an
example .flt file (although it doesn't use "-" exclude directives).
Moved the UI active flag from the machine to the UI manager. Nothing
else uses it anyway (it's still accessible to scripts).
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