Bug 4004 partial: Fix segfault via Ftp::Client::readControlReply

Bug 4004 partial: Fix segfault via Ftp::Client::readControlReply

Added nil dereference checks for Ftp::Client::ctrl.conn, including:
- Ftp::Client::handlePasvReply() and handleEpsvReply() that dereference
  ctrl.conn in DBG_IMPORTANT messages.
- Many functions inside FtpClient.cc and FtpGateway.cc files.

TODO: We need to find a better way to handle nil ctrl.conn. It is only
a matter of time when we forget to add another dereference check or
discover a place we missed during this change.

Also disabled forwarding of EPRT and PORT commands to origin servers.
Squid support for those commands is broken and their forwarding may
cause segfaults (bug #4004). Active FTP is still supported, of course.

This is a Measurement Factory project