Backout the linux aio stuff. Several problems where identified and the

Backout the linux aio stuff. Several problems where identified and the
dynamic nature (if no native aio code is available, the linux part
returns ENOSYS because of missing requisites) should be solved differently
than it is.

All this will be done in P4.

Not included in this commit is a backout of the changes to the native aio
code (removing static in some places). Those changes (and some more) will
also be needed when the reworked linux aio stuff will reenter the tree.

Requested by:	rwatson
Discussed with:	rwatson