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>
Fix: add UST context in the same order the user enabled them

This is to avoid having different field order in different traces for
the same contexts enabled in the same order by the user.

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