Added ftp_client_idle_timeout directive to squid.conf.

Added ftp_client_idle_timeout directive to squid.conf.

Many FTP clients do not deal with control connection closures. They cannot
retry pconn races without asking the user for credentials, for example.  Thus,
the existing client_idle_pconn_timeout default (2 minutes) does not work well
for FTP clients. One the other hand, increasing that may create too many idle
HTTP connections for Squid to maintain.

The new timeout is specific to ftp_port traffic. It does not affect ftp://...
requests sent to an http[s]_port.