Add WWW-Authenticate to 401 responses

Per http://www.ietf.org/rfc/rfc2616.txt, when a 401 error is returned, the
Www-Authenticate response header MUST also be returned. The format is
described in http://www.ietf.org/rfc/rfc2617.txt.

Swift supports and/or implements a number of authentication schemes
including tempauth, Keystone, tempurl, formpost and container sync. In
this fix, we use a catch-all, "Swift". The realm is the account (where
known) or "unknown" (bad path or where the 401 is returned from code
that does not have the request). Examples:

     Www-Authenticate: Swift realm="AUTH_1234567889"
     Www-Authenticate: Swift realm="unknown"

Fixes bug #1215491

Change-Id: I03362789318dfa156d3733ef9348795062a9cfc4
Merge "Add WWW-Authenticate to 401 responses"