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/meson.build

@@ -113,6 +113,7 @@ if host_machine.system() != 'windows'
     'socket-address',
     'stream-rw_all',
     'unix-fd',
+    'unix-mounts',
     'unix-streams',
     'g-file-info-filesystem-readonly',
     'gschema-compile',