Limit the number of allowed X-Forwarded-For hops (#1589)

Limit the number of allowed X-Forwarded-For hops (#1589)

Squid will ignore all X-Forwarded-For elements listed after the first 64
addresses allowed by the follow_x_forwarded_for directive. A different
limit can be specified by defining a C++ SQUID_X_FORWARDED_FOR_HOP_MAX
macro, but that macro is not a supported Squid configuration interface
and may change or disappear at any time.

Squid will log a cache.log ERROR if the hop limit has been reached.

This change works around problematic ACLChecklist and/or slow ACLs
implementation that results in immediate nonBlockingCheck() callbacks.
Such callbacks have caused many bugs and development complications. In
clientFollowXForwardedForCheck() context, they lead to indirect
recursion that was bound only by the number of allowed XFF entries,
which could reach thousands and exhaust Squid process call stack.

This recursion bug was discovered and detailed by Joshua Rogers at
https://megamansec.github.io/Squid-Security-Audit/xff-stackoverflow.html
where it was filed as "X-Forwarded-For Stack Overflow".