Initial native FTP Relay support.

Initial native FTP Relay support.

* Added ftp_port directive telling Squid to relay native FTP commands.
* Active and passive FTP support on the user-facing side;
  require passive connections to come from the control connection src IP.
* IPv6 support (EPSV and, on the user-facing side, EPRT).
* Intelligent adaptation of relayed FTP FEAT responses.
* Relaying of multi-line FTP control responses using various formats.
* Support relaying of FTP MLSD and MLST commands (RFC 3659).
* Several Microsoft FTP server compatibility features.
* ICAP/eCAP support (at individual FTP command/response level).
* Optional "current FTP directory" tracking (cannot be 100% reliable due to
  symbolic links and such, but is helpful in some common use cases).
* FTP origin control connection is pinned to the FTP user connection.
* No caching support -- no reliable Request URIs for that (see above).
* Significant FTP code restructuring on the server-facing side.
* Initial steps towards HTTP code restructuring on the client-facing side.

See merged revisions commit log for details.