GIOLister: ignore system mounts as defined by GIO

Strawberry has some heuristics to exclude things
like the root mount, /boot, tmpfs, etc. from the devices list.

However, it's somewhat incomplete. GIO (GLib component)
has more complete definition of "internal system mounts"
that should not be presented to the user.

Additionally, don't try to query filesystems free/total size
if it's internal, as that triggers automounting for autofs
filesystems (#410).

src/config.h.in

@@ -7,6 +7,7 @@
 
 #cmakedefine HAVE_BACKTRACE
 #cmakedefine HAVE_GIO
+#cmakedefine HAVE_GIO_UNIX
 #cmakedefine HAVE_DBUS
 #cmakedefine HAVE_X11
 #cmakedefine HAVE_UDISKS2