Stop using small values to back manifest constants.

Zero is an exception; we do still use zero.  But for values the API
does not promise anything about except that they can be stored in an
int and, sometimes, their sign - those, we now use values further from
zero, values less likely to occur by accident.  We also use distinct
values across the ABI; for example, it is now no longer the case that
AIO_WRITEV_ERROR equals AIO_OQ_HL_EMPTY.