Add a (very evil) async IO framework. It doesn't do much - just get

Add a (very evil) async IO framework. It doesn't do much - just get
the calls in there. The next (few) commits will be the async code.

Yes, yes, I know it should be "Generic", but I'm trying to get COSS
to "user testing" state, so this is the best 'comprimise'.

For now, COSS will use the libaio routines (or the sync stuff in
disk.c - the calling API is the same so wrapping stuff in an #ifdef
will be quite easy) which are available on Solaris, FreeBSD
and possibly Linux.