dbus: Reuse connections in registerService

Instead of creating a new session/system connection every time a
service is registered, try to reuse those that may have been
previously allocated.

This fixes in particular a bug that occurred when registering
AND calling an interface on the same bus (session/system)
in the same program.
Merge pull request #81 from Orange-OpenSource/reuse_cnx

dbus: Reuse connections in registerService