Add URL-rewriter based on local file existence

Add URL-rewriter based on local file existence

Not exactly new. The basic code for this has been bundled with Squid in
contrib/ since 1996.

* Shuffle the contrib/rredir.* helpers code into helpers/url_rewrite/LFS/

* Add missing copyright boilerplate, CONTRIBUTORS and CREDITS entries

* Add Makefile support to install the perl helper

Redesign perl helper:

* update for strict Perl syntax

* update to current (Squid-3.4+) helper protocol

* replace hard-coded rewriter config with flexible command-line
  parameters

* add debug support

* add concurrency support

* add helper documentation