Polish ZPH configuration interface

This completes the ZPH patch merge for Squid 3.1. It makes significant
changes to the squid.conf options previously used.

ZPH controls are now formally called "qos_flows" with a set of options
to set individual output flow TOS based on the flow source. see squid.conf
documentation for details.

Since this is new IP-layer code it also places the new QoS code in a
directory src/ip/ and library libip.la inline with the planned source
layout model.