Initial support for gatewaying FTP EPRT and EPSV commands.

Initial support for gatewaying FTP EPRT and EPSV commands.

Client side validates the IPv6-friendly commands, but the server side
independently decides which of the four standard data connection establishment
commands to use. In practice only PASV and EPSV commands are sent because
Squid still does not support active FTP connections on the server side.

Moved EPSV- and some PASV-handling code from proxy-specific ftp.cc into
general FtpServer.cc for the client-side gateway code to reuse.