GMappedFile: return an error when trying to map a device

Previously, we were returning an empty buffer for all filenames
where fstat() gives a size of 0. But this is only appropriate
for regular files.

Also improve the documentation around this issue. Based on a
patch by Ryan Lortie.

Conflicts:

	glib/tests/mappedfile.c

https://bugzilla.gnome.org/show_bug.cgi?id=659212

glib/tests/mappedfile.c

@@ -36,6 +36,18 @@ test_empty (void)
   g_mapped_file_free (file);
 }
 
+static void
+test_device (void)
+{
+  GError *error = NULL;
+  GMappedFile *file;
+
+  file = g_mapped_file_new ("/dev/null", FALSE, &error);
+  g_assert_error (error, G_FILE_ERROR, G_FILE_ERROR_INVAL);
+  g_assert (file == NULL);
+  g_error_free (error);
+}
+
 static void
 test_nonexisting (void)
 {