Rework sshd enable root login questions in light of sshd PermitRootLogin

Rework sshd enable root login questions in light of sshd PermitRootLogin
default change. The new default is not to ask to enable root logins
when a non-root user has been addedi. There is some additional sublety
for auto-installs that provide root ssh keys.

patch by myself and rpe@ with feedback from sthen@;
ok rpe@ deraadt@ sthen@