[PATCH] Server PAC file from Squid

One of the old requests is to server the PAC files directly from Squid
without needing a local HTTP server just for the one file.

This patch makes Squid intercept requests which use it's visible_hostname
(and hostname_aliases) for /wpad.dat and pass out a PAC file specified in
the configuration.

This is particularly useful for public hot-spot installs. Allowing tighter
security since unauthorized and badly configured clients no longer need
access to anywhere but the proxy itself; and less configuration since an
additional web server is no longer required just to serve the PAC file.

NP: An ERR_AGENT_PACFILE error template is added to display configuration
details. For now, like the manual configuration template an assumption has
to be made about the Squid listening ports since we don't yet have any to
dynamically insert the listening port into error pages.

Amos