Introduce configure --with-lttng-system-rundir

Allow overriding the path where the system-wide sessiond keeps its
runtime files.

It does _not_ allow multiple instances of system-wide sessiond to run in
parallel though, because the wait shm files still requires having only
one single system-wide sessiond at any given time.

Fixes #521

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>