[MERGE] http_port tproxy options MUST imply transparent.

Initial hack at TPROXY v4 support in Squid-3

This patch removes the --enable-linux-tproxy configure option and replaces
it with two alternatives --enable-linux-tproxy2 and --enable-linux-tproxy4

*-tproxy2 performs the original tproxy behaviour of squid.

*-tproxy4 adds and uses the new IP_TRANSPARENT socket option available
in kernel 2.6.26 for the netfilter TPROXY target.

TODOs:
        - need to make TPROXY and REDIRECT targets dual-operable
          (that will make --enable-linux-netfilter =~= --enable-linux-tproxy4)
        - still need to make squid bind outbound with client address
        - maybe make IPInterception.* a class to handle transparent API