gvfs-open: Fix incorrect use of g_app_info_launch_default_for_uri()
gvfs-open: Fix incorrect use of g_app_info_launch_default_for_uri()

First, as we never check whether the passed arguments are indeed URIs,
we'd be failing to work correctly for relative paths.
Secondly, we would be running g_app_info_launch_default_for_uri() twice
for no good reason.
Finally, we were leaking the output of g_file_get_uri()

https://bugzilla.gnome.org/show_bug.cgi?id=738690

gicmo/gvfs,gicmo/gvfs,gicmo/gvfs,gicmo/gvfs