Add initialPassword and initialHashedPassword options

These are like password and hashedPassword, except that they only
apply when the user is initially created.
Handle removing a password if mutableUsers = false
Handle initialPassword and initialHashedPassword for !mutableUsers

In this case, they're equivalent to setting ‘password’ and
‘hashedPassword’ (since there is no distinction between an initial and
non-initial user account state).
Obsolete security.initialPassword

You can now set users.extraUsers.root.initialHashedPassword instead.