This patch implements the Linux's timerfd_*() system calls, declared in
<sys/timerfd.h>. These define a file descriptor, usable for read() or
poll() and friends, which becomes readable when a timer expires.

This aspires to be a full implementation of timerfd, with all the intricate
details explained in timerfd_create(2).

timerfd was added to Linux five years ago (Linux 2.6.25). Boost's asio,
in particular, uses this feature if it thinks it is available.

Fixes #129.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>