sshd: spawning a session on behalf of a specific user doesn't work

[treeswift]

CreateProcessW works, but CreateProcessAsUserW doesn't work even if the user handle passed is of the current user running sshd.exe, freshly authenticated.

Use cases to support (test/debug/test):

• system service account, incoming session on behalf of a named user
• named user account, incoming session on the same user's behalf
• named user account (admin), incoming session on another user's behalf