vsftpd: change default secure_chroot_dir

Change default value of secure_chroot_dir to /var/run/vsftpd/empty, add
volatiles entry for it, to ensure it won't fail to start with error:
"500 OOPS: vsftpd: not found: directory given in 'secure_chroot_dir':/var/share/empty"

This shows up in both standalone mode or started by xined.

Signed-off-by: Ming Liu <ming.liu@windriver.com>
Signed-off-by: Joe MacDonald <joe@deserted.net>