bug#21318: Only the first 8 characters of passwords are significant

[Mark H Weaver]

yenda on #guix reported that when typing user passwords, only the first
8 characters need to be typed correctly to successfully log in.

DusXMT on #guix mentioned that [GNU/]Linux From Scratch instructs users
to change "#ENCRYPT_METHOD_DES" to "ENCRYPT_METHOD_SHA512" in
etc/login.defs:

  http://www.linuxfromscratch.org/lfs/view/stable/chapter06/shadow.html

I tried modifying both /etc/login.defs and etc/login.defs in our
'shadow' package recipe, and then tried updating my password entry with
'passwd' but it still only pays attention to the first 8 characters.

'strace' reveals that 'passwd' doesn't even look for any file named
"login.defs".

I'm not sure what's going on here, but it would be good to fix it soon.

     Mark