[PATCH] "enum Direction" for auth credentials request state

The state of credentials lookup and handling is recorded by
authenticateDirection / AuthUserRequest::direction() and its per-scheme
helper methods AuthUserRequest::module_direction().

This formalizes and coordinates the state being returned by using a
shared enum.

The states can generally be considerd as:
  - LOOKUP with a helper still needs to validate the credentials
  - CHALLENGE if the helepr needs more info from the client
  - VALID if everything is fine and the credentials are known Good/Bad
  - ERROR if there is any problem with the state or credentials

This combination has highlighted a few strange things in the NTLM and
Negotiate states. Where known but Failed credentials are marked as
ERROR. This needs closer investigation since it should logically be a
re-CHALLENGE in all auth types.

Also there is a little obfuscation of the cases around the generalized
fixHeader() calls. This will be handled in a followup patch.

On the side any ideas what to call this state management besides
"Direction"? I'm at a loss for terminology.

  NP: labels above are relatively arbitrary. If they need changing to
fit some terminology scope of the function name so be it.

Amos

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