add muidlsupp.h from µiX

It should be cleaned up a bit. For instance there's no need for
<kernel/types.h> in it at all, and the file's licensing could be something
completely different.
rename muidl.h -> defs.h, rework muidlsupp.h

Now the µIDL support interface is in <muidl.h>, as it should be, and µIDL's
definitions and macros (of which there are too many) are in defs.h.
add more stuff to <muidl.h>

This crap was spread all over the µiX tree. The bits added in this cset are
the genuinely useful ones.