<arch>-protos.h, why?

[Stan Shebs]

OK, this is probably a dumb question, but why do we have separate
<arch>-protos.h files?  As far as I can tell by digging around,
they're invariably included after <arch>.h, which would suggest
that the prototypes could be included in <arch>.h directly.  So
presumably there is some other reason for splitting them out.

I also note that although they've become commonplace, there's
no mention in any of the porting documentation.  If there's a
rule now, surely it's worth a mention?

(This is an issue for me because multi-platform Darwin support
needs prototypes, but they don't fit neatly into the current
system, at least as far as I understand it.)

Stan