vchoi_fork

Split VFD_SWMR_TEST_FOR_END_OF_TICK() into VFD_SWMR_ENTER() and
VFD_SWMR_LEAVE() for use by FUNC_ENTER_API/_LEAVE_API macros.  In the
macros, don't HGOTO_ERROR(), since that will jump back to the `out`
label, but HDONE_ERROR() on error, instead.