Daemonize sessiond on `lttng create`

Daemonize sessiond on `lttng create`

Since the session daemon forked by `lttng create` shares its
standard output/error FDs when not using `--daemonize`, redirecting
the standard output/error of this command to another program "hangs"
because the session daemon never terminates.

Example that's not working (when sessiond is not running):

    lttng create | wc

or:

    lttng 2>&1 | wc

Using sessiond's `--daemonize` option makes it close its FDs. This
option also ensures that when the sessiond process exits, it has forked
itself as a daemon and is ready to accept commands. Therefore we don't
need to catch SIGCHLD and SIGUSR1; just waitpid() on sessiond's PID and
make sure it exited normally and with an exit status of 0 to continue.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>