Support creds passing between lttng and sessiond

From now on, the lttng command line sends it's credential at each
command along with the regular communication data.

The tracing output directory is set to those credentials (uid/gid) if
the session daemon is running as a regular user (even if the user is in
the tracing group). So *no* tracing group is ever set if the session
daemon is not running as a non-root user.

If the sessiond is running as root, the tracing group is set (if exist)
and the uid of the calling user is used.

This fixes the triple enable-channel/event needed to trace UST
applications with a non-root session daemon. This was reported by
Yannick Brosseau <yannick.brosseau@polymtl.ca>.

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