With one exception, "hardfault" is used like a "bool".  Change that
exception and make it a "bool".

The "lookup_still_valid" field is used as a "bool".  Make it one.

Convert vm_fault_hold()'s Boolean variables that are only used
internally to "bool".  Add a comment describing why the one
remaining "boolean_t" was not converted.

Merge and sort vm_fault_hold()'s "int" variable definitions.