Fix: Don't dispatch uninitialized hash tables to the ht_cleanup thread

Some error-handling code paths may call ht_cleanup_push() with NULL hash
tables which will crash the clean-up thread.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>
Fix: Duplicate lttng-sessiond thread creation comments

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>
Fix: typo in --sig-parent help

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