Stop using external libaio.h.

Stop using external libaio.h.

This change incorporates a partial copy instead
of using external libaio.h.
Why?
Because we want to properly decode 32-bit aio calls
by 64-bit strace. For that, we need more definitions than
libaio.h provides.
(These defs are not done yet, but will eventually be done).
Keeping our local 32-bit compat defs in sync with libaio.h
_without seeing libaio structs_ is hard/more bug prone.
A smaller benefit is that we don't need libaio installed.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>