Boilerplate: update authorship contributions by Markus Moeller

Boilerplate: update authorship contributions by Markus Moeller

 With permission.

./helpers/negotiate_auth/kerberos/COPYRIGHT
- remove, since it duplicates the Squid top level COPYRIGHT file.

./lib/base64.c
- removing the inaccurate line "AUTHOR: Markus Moeller", since other
  authors (myself at least) have adjusted that code as well.

./src/peer_proxy_negotiate_auth.h
- removing the line: "AUTHOR: Markus Moeller (markus_moeller at
  compuserve.com)"

./src/peer_proxy_negotiate_auth.cc
- removing the "Hosted at" line as inaccurate, the file is hosted in
  Squid repository, launchpad and elsewhere.

- replacing the license blurb with Squid Software Foundation blurb
  (remains GPLv2+).