Change SIGCHLD to SIGUSR1 when lttng waiting on sessiond

Using lttng cli, if the session daemon dies after exec(), a SIGCHLD is
returned. It was catched but not handled to cleanly quit and inform the
user that the session daemon failed.

We now use SIGUSR1 to signal the lttng cli that the session daemon is
ready to receive command.

Signed-off-by: David Goulet <dgoulet@efficios.com>