Add the load session API call

Introduce lttng_load_session(attr) and some setter/getter for the
attribute object.

Signed-off-by: David Goulet <dgoulet@efficios.com>
Add hidden attribute to some common functions

Signed-off-by: David Goulet <dgoulet@efficios.com>
Add missing test files to EXTRA_DIST

Signed-off-by: David Goulet <dgoulet@efficios.com>
Update version to v2.5.0-rc1

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