Cache Manager migration support

Cache Manager migration support

 * Add a little bit of XHR script to the CGI cachemgr front page which
probes each of the managed proxies for http:// and https:// capabilities
and produces web links to their internal managers.

 * Reserve the template name MGR_INDEX for use by cachemgr scripts.
But do not distribute any preset template. This allows manager apps to
provide their own static template with linked scripts and objects.

 * The error page system is updated to create a blanket message
indicating missing template instead of aborting Squid if a template is
not even installed.