Adding aio.h and mqueue.h

[Joel Sherrill]

Hi

We have migrated almost all of our POSIX defined header files from RTEMS
itself to newlib.  I think aio.h and mqueue.h are the last POSIX headers
left on our side.

What's the feeling on merging these? Would it be ok to put them in
newlib/libc/include?

Our version of the headers have Doxygen comments. Would these be OK in
newlib or do I need to remove the Doxygen?

Thanks.

--joel