aio / timers: Add prctl(PR_SET_TIMERSLACK, 1, ...) to reduce timer slack

Where supported, called prctl(PR_SET_TIMERSLACK, 1, ...) to
set one nanosecond timer slack to increase precision of timer
calls.

Signed-off-by: Alex Bligh <alex@alex.org.uk>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>