This will make a number of things easier in the future, as well as (finally!)
avoiding the Id-smashing problem which has plagued developers for so long.
Boy, I'm glad we're not using sup anymore. This update would have been
insane otherwise.
password: ask for it, but don't tell that S/key password required.
It looks like non-s/key system from outside.
Additionally tell that s/key required when it is so for normal case
It happens if 1) regular passwords not allowed, 2) skey database
not activated for given user.
Under some rare circumstanes skey_challenge can return empty
diagnostic or even previous buffer, fix it.
