Add a -l/--load option to the session-daemon

This option makes it possible to set a PATH from which session
configurations are loaded by the session daemon on startup.

Usage: lttng-sessiond -l PATH

The session configuration file found at PATH, or all those present in it
if it is a directory, will be loaded.

All sessions found under the user's session configuration directory and
under the system session configuration directory are now loaded by
default.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>