auth-source patch for secure logging

[Ted Zlatanov]

Attached is a patch to auth-source.el that:

- introduces auth-source-debug

- sets it to nil by default, so no logging is done (this changes the
  previous behavior where we always log to *Messages*)

- allows t (relay to 'message) and any function as options for that
  variable

It's against the Gnus CVS, but Emacs CVS has the same contents.

If it's possible to include this in the upcoming release, I think it
would improve security for Emacs users.  It's not a critical fix,
however, so I will defer to the maintainers to decide.

If it's accepted, please commit it to the Emacs CVS and then Miles can
sync it back to the Gnus CVS.

Ted