Add support for O_ATOMIC

O_ATOMIC makes writes atomic, meaning that they are fully stable
on media (in the event of a power cut) when acknowledged by the
device and OS.

This only truly works on Linux with the pending patches to
add O_ATOMIC.

Updated by Jens to:

- Add man page and HOWTO description of the option
- Make O_ATOMIC imply O_DIRECT, so that it actually works if you
  don't set O_DIRECT manually.
- Add the option to the conversion list so it works for
  client/server.
- Error handling so that if atomic=1 is set and the OS does not
  support it, error out instead of just pretending it works.

Signed-off-by: Jens Axboe <axboe@kernel.dk>