Allow vsftpd local account change passwd

1. set /usr/bin/passwd as shell of local ftp account.

2. user then ssh or telnet to vsftp server will get prompt to change passwd

#remember add /usr/bin/passwd into /etc/shells, or ftp account will get "530 login incorrect".

#"check_shell=No" in vsftpd.conf is useless since it only work whil vsftpd compiled in "non-pam" option, however mostly RHEL/CentOS versions pack vsftpd rpm with pam option? at least I verified my CentOS 5.3 setup and found check_shell is not work in my environment.