ttyd: force enable authentication for login

ttyd: force enable authentication for login

Currently, we called `/usr/libexec/login.sh` as login command, but unfortunately the auth
is disabled by default in it[1], and this is really serious as it could be a free "backdoor"
for any spoiler who has conntectd to the router via LAN or wireless.

In my option, it shouldn't be exposed to anyone without auth, so I set the default login
command to `/bin/login`. And for those who really want that, they can do it themselves.

1. `login.sh` adjusts whether use authentication or not from system config named ttylogin,
which is set to disabled by default. See package/base-files/files/bin/config_generate#L243.

Signed-off-by: Tianling Shen <cnsztl@project-openwrt.eu.org>