Add the notify UST application scheme

This commit introduce the use of a wait shm futex that is used to notify
applications that the session daemon is running and ready to accept
registrations. See shm.c/.h for those calls and the new
futex_wait_update() in futex.c. This scheme is also found in lttng-ust
2.0 and mainly taken from there.

Also, enable-channel of the lttng client was modified to accept UST PID
domain. It's the first phase for testing UST features of the
liblttngctl.

Signed-off-by: David Goulet <david.goulet@polymtl.ca>