Add support for UST application registration

This is a big commit ;)

Introduce two new threads and one almost rewritten. The first thread
added is the one that manage UST registration. This thread is *very*
lightweight and, basically, receive the registration request, queue it
in a lock free queue and wake up the dispatch thread (second new thread)
using a N wakers / 1 waiters futex scheme.

This dispatch thread will then dequeue and notify the application thread
(that manages application registration and monitor UST sockets). The
notification consist of sending the dequeued node (command) to the
application thread pipe where the application thread is blocked on
poll().

For now, the registration is handle meaning that the application will
get added to the session daemon internal data structures, the daemon
will send a REGISTER_DONE command to the application and, finally, end
the communication by waiting for the UST reply of the previous command.

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <david.goulet@polymtl.ca>