lttng-sessiond: keep enabled/active state for sessions, cleanup enabled state for...

allows

lttng list
Available tracing sessions:
  1) test1 (/root/lttng-traces/test1-20111115-191312) [inactive]
  2) test2 (/root/lttng-traces/test2-20111115-191310) [active]

and

root@thinkos:~# lttng list test2
Tracing session test2: [active]
    Trace path: /root/lttng-traces/test2-20111115-191502

When events/channels/sessions enabled state is set to -1, show an empty
string instead of the meaningless "[enabled: -1]".

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