Draft: monitor: Do not load monitors when GIO_USE_VFS=local

Various applications use GIO_USE_VFS=local to prevent starting gvfs daemons. However, the gvfsd and gvfsd-fuse processes are started anyway when GVFS_REMOTE_VOLUME_MONITOR_IGNORE is not set as well. This is because the g_vfs_get_default call causes all GIO modules to be loaded. Consequently, the gvfsd daemon is started to list available volume monitor implementations since the commit 2592a8ed. Another problem is that also non-native volume monitors are listed. Let's simply don't load our monitors when GIO_USE_VFS=local is set and rely on the unix volume monitor.

Related: #696