From: Manfred Spraul <manfred@colorfullife.com>

SIGEV_THREAD means that a given callback should be called in the context on a
new thread.  This must be done by the C library.  The kernel must deliver a
notice of the event to the C library when the callback should be called.

This patch switches to a new, simpler interface: User space creates a socket
with socket(PF_NETLINK, SOCK_RAW,0) and passes the fd to the mq_notify call
together with a cookie.  When the mq_notify() condition is satisfied, the
kernel "writes" the cookie to the socket.  User space then reads the cookie
and calls the appropriate callback.