proftpd: use /bin/false as the login shell and add home-dir

Use /bin/false as the login shell, just like what Ubuntu does,
otherwise there might be secure issue; add /var/lib/ftp as user
ftp home-dir.

Signed-off-by: Roy Li <rongqing.li@windriver.com>
Signed-off-by: Joe MacDonald <joe@deserted.net>