Determine appropriate salt for invalid users.

When sshd is processing a non-PAM login for a non-existent user it uses
the string from the fakepw structure as the salt for crypt(3)ing the
password supplied by the client.  That string has a Blowfish prefix, so on
systems that don't understand that crypt will fail fast due to an invalid
salt, and even on those that do it may have significantly different timing
from the hash methods used for real accounts (eg sha512).  This allows
user enumeration by, eg, sending large password strings.  This was noted
by EddieEzra.Harari at verint.com (CVE-2016-6210).

To mitigate, use the same hash algorithm that root uses for hashing
passwords for users that do not exist on the system.  ok djm@
