add UNOP_AUX OP class

add UNOP_AUX OP class

This is the same as a UNOP, but with the addition of an op_aux field,
which points to an array of UNOP_AUX_item unions.

It is intended as a general escape mechanism for adding per-op-type extra
fields (or arrays of items) to UNOPs.

Its class character (for regen/opcodes etc) is '+'.

Currently there are no ops of this type; but shortly, OP_MULTIDEREF will
be added, which is the original motivation for this new op type.