bus: always explicitly close bus from main programs

bus: always explicitly close bus from main programs

Since b5eca3a2059f9399d1dc52cbcf9698674c4b1cf0 we don't attempt to GC
busses anymore when unsent messages remain that keep their reference,
when they otherwise are not referenced anymore. This means that if we
explicitly want connections to go away, we need to close them.

With this change we will no do so explicitly wherver we connect to the
bus from a main program (and thus know when the bus connection should go
away), or when we create a private bus connection, that really should go
away after our use.

This fixes connection leaks in the NSS and PAM modules.