'bool' is preferred to 'boolean_t'. We only get the boolean_t
definition by header pollution (though the same is true for
bool). Since we use both, switch entirely to bool.
Note: We still have TRUE/FALSE instead of true/false in heavy use in
the rest of the file. These are with ints of various flavors, so
that's appropriate, even though we should eventually migrate to bool
and true/false (though the tables they are in are nicely packed with
short and wouldn't be so nicely packed with bool, another reason
to leave it alone for now).
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