Update and fix UST register and session creation

The handling of the apps command pollfd, in the registration thread and
the update function, was incorrect. The update process, when
unregistring, was overwritting connected socket thus not being able to
monitor them with poll().

This is the first step forward to fully support LTTng UST tracer.

Add create UST session to ustctl and the session daemon. At this
commit, this feature is not used yet.

Move the register done command to ustctl. Adds the ustcomm_send_command
which returns the reply structure from the UST application.

The ustctl and ustcomm are cleaned up.

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