glocalfilemonitor: Fallback to poll file monitor for NFS

GLib currently tries to use FAM volume monitor for monitoring files within home on NFS. If FAM support is not available, it fallbacks by default to GInotifyFileMonitor. I think we should fallback to GPollFileMonitor instead, because inotify is not reliable on NFS, which may cause issues for dconf. With this patch, it should be safe to not build libgiofam and still be sure that dconf works properly if home is mounted on NFS. I think this might be a first step to remove FAM support from GLib completely, because gamin is buggy and dead for several years already. Gamin just polls files on NFS anyway. This change applies on files only, because GPollFileMonitor seems doesn't support dirs, however it should be enough for dconf. The other drawback is that one can't set poll timeout currently. Just a note that this can still be overwritten by GIO_USE_FILE_MONITOR.