Replace the macro RETURNX() with its expansion in FT_RETURN_{FALSE,TRUE}

Replace the macro RETURNX() with its expansion in FT_RETURN_{FALSE,TRUE}

The macros FT_RETURN_FALSE and FT_RETURN_TRUE in pp_ctl.c are already very
complex, sufficient to trigger an internal failure in HP's compiler [and
possibly also some humans :-)]. Replacing RETURNX() with the 3 statements it
expands to makes the intent of macros clearer, and exposes more refactoring
possibilities.