A number of places in the code need to get a connection to NM through
D-Bus, and that connection could be either a shared bus connection or
a private connection.  Consolidate that logic.

Unfortunately only dbus-glib >= 0.100 correctly supports private
connections (commit 8b7e4a1c4ae055864e26db4addbcc2dc38ee6963 fixes
this problem) so the private connection functionality is not enabled
for older dbus-glib versions.