sessiond: refactoring: introduce start/stop_kernel_session

sessiond: refactoring: introduce start/stop_kernel_session

The clear feature will need to stop kernel session as well, so move this
code to its own function rather than duplicate it.

start_kernel_session already exists, but is static. Expose it so it
can be used from a separate compile unit.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I68da1ee2e51105278265866590734c320a9789bd
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>