Add vfs_aio_fork

Add vfs_aio_fork

This is used for two purposes:

First, I'm using it to test the async I/O code. In the forked process it is
pretty easy to delay a reply for a random amount of time. See the
BUILD_FARM_HACKS snippet.

Second, there are systems around that claim to have Posix AIO but which is
broken. This might be some help for those systems.

Also add tests how to pass file descriptors
(This used to be commit b0e52cecf2009d4c7f29412dadf17910e54e4327)