Smarter auth_param utf8 handling, including CP1251 support (#480)

Smarter auth_param utf8 handling, including CP1251 support (#480)

When forwarding authentication credentials to authentication helpers:

* With auth_param utf8 parameter: Squid assumed that the received
  credentials are encoded with Latin-1 (and re-encoded them with UTF-8).
  This assumption is wrong for Internet Explorer running with CP1251
  regional settings. Now Squid uses HTTP Accept-Language request header
  to guess the received credentials encoding (Latin-1, CP1251, or UTF-8)
  and converts the first two encodings into UTF-8.

* Without auth_param utf8 parameter: No changes. Squid sends credentials
  in their original encoding, only applying RFC 1738 escaping on top.

Chrome and Firefox should not be affected because they always use UTF-8
encoding when sending authentication credentials.