Implement GVFS service registry

[chrisd8088]

The gvfs mount command wants to register each new mount with a system service daemon, presumably so that the mounts can be auto-mounted upon restarted. We will need to consider how best to implement such a GVFS.Service daemon for Linux.

Note that in the interim, we are using the GVFS_UNATTENDED environment variable to disable the gvfs command's attempts to contact the GVFS.Service process.