localfile: add support for monitoring on NFS

localfile: add support for monitoring on NFS

Add a pair of new extension points: 'gio-nfs-file-monitor' and
'gio-nfs-directory-monitor'.

Add a check to GLocalFile when creating a file monitor.  If the
requested file is in the user's home directory and the user has an NFS
home directory then attempt to use an implementation of one of the new
extension points.  If we don't have any implementations then fall back
to the normal "local" monitors.

https://bugzilla.gnome.org/show_bug.cgi?id=592211