C++ compilable headers

This patch:
http://www.squid-cache.org/~robertc/squid-cppcleanup.patch.bz2

cleans up the struct members to allow the use of a C++ compiler to build
squid. it also detects the users C++ compiler, but it doesn't change any
of the source files to be C++.

It's makes no change to the behaviour of any squid internals, and
doesn't affect code mix or style (beyond the renaming of some struct
members). So it's orthogonal to the discussion about allow C++ into the
current codebase, but very useful if someone wants to experiment.

I intend to commit this in a week if no feedback/discussion occurs.

Cheers,
Rob