[PATCH] icp clean-up and ipv6/ipv4 related fixes

- The "Sender Host Address" field of the ICP messages header it is a
32bit integer so it can be only an ipv4 ip address. Moreover according
the ICP RFC:
"Sender Host Address
   The IPv4 address of the host sending the ICP message. This field
   should probably not be trusted over what is provided by getpeer-
   name(), accept(), and recvfrom(). There is some ambiguity over
   the original purpose of this field. In practice it is not used."

This patch in the case we have an IPv4 address set this header field
with the IPv4 address else set it to 0.

- Remove the echo_hdr static variable from neighbors.cc file and the
theIcpPublicHostID variable from the icp_v2.cc file. They are part of
the old "source_ping" squid feature code which does not exit any more.