gvfs: allow gvfs-network to access it's gconf schemas

gvfs-network fails to start until it stores some setting in Gconf (memory
backend is used by default). Unfortunately, it needs schemas for to work
correctly. By default, glib searches for schemas in /usr/share/glib-2.0/schemas
OR under GSETTINGS_SCHEMA_DIR. This patch sets this variable to let gvfs
find it's precious.