[PATCH] Limit X-Forwarded-For growth

Hello,

    Should the attached patch go in? It prevents assertions (or worse)
during forwarding loops or carefully crafted messages. Production-tested
in Squid 3.0, although I do not know whether the code was ever triggered
outside the lab.

This change also prevents most cases of pointless computation of the
original X-Forwarded-For value list in Squid 3.1. That computation can
be quite expensive.

Thank you,

Alex.