gio: Add API for identifying system FS types and device paths

This is needed by gnome-control-center and gnome-settings-daemon; it
makes existing checks from gunixmounts.c public.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=788927

gio/tests/Makefile.am

@@ -266,6 +266,7 @@ test_programs += \
 	socket-address				\
 	stream-rw_all				\
 	unix-fd					\
+	unix-mounts				\
 	unix-streams				\
 	g-file-info-filesystem-readonly		\
 	$(NULL)