parse credentials and challenges

parse credentials and challenges

* module/web/http.scm (parse-credentials, validate-credentials)
  (write-credentials, parse-challenge, parse-challenges)
  (validate-challenges, write-challenge, write-challenges)
  (declare-credentials-header!, declare-challenge-list-header!): New
  helpers.
  ("Authorization", "Proxy-Authorization"): Parse out credentials.
  ("Proxy-Authenticate", "WWW-Authenticate"): Parse out challenges.