libaio: resurrect aio poll and add io_pgetevents support

[Christoph Hellwig]

Hi all,

this series resurrects IOCB_CMD_POLL support and adds support for the
new io_pgetevents system call, as well as adding a test case.

Git branch:

    git://git.infradead.org/users/hch/libaio.git aio-poll

Gitweb:

    http://git.infradead.org/users/hch/libaio.git/shortlog/refs/heads/aio-poll