cgmanager: avoid stray dbus connections

cgmanager: avoid stray dbus connections

There are two parts to this fix.

First, create a private DBusConnection manually, instead of using
nih_dbus_connect.  The latter always creates a shared connection,
which cannot be closed.  Note: creating an actual shared connection,
mutexing it among all threads, and creating per-thread proxies would
be an alternative - however we don't want long-lived connections as
they tend not to be reliable (especially if cgmanager restarts).

Second, use pthread_setspecific to create per-thread keys which can
be associated with destructors.  Specify a destructor which closes
the dbus connection.  If a thread dies while holding cgmanager,
the connection will be closed.  Otherwise, we close the connection
and unset the key.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>