Bug 2862: add http(s):// support to cache manager

Bug 2862: add http(s):// support to cache manager

via http://$visible_hostname/ requests intercepted by the internal
server feature. Also https:// if SSL/TLS is available on the receiving
port.

In order to safely identify the manager reports the path prefix
/squid-internal-mgr/ is added. The old cache_oject:// scheme format
paths follow that identifier prefix.

To retrieve pages the proxy visible_hostname, management port (first
forward-proxy port), and the path prefix must all be present in the URL.

The "manager" ACL is altered to url_regex in order to match the new
protocol+path URL syntax.

Unlike the cache_object:// scheme, http[s]:// do not accept password
as part of the URL. If one is needed it must be sent via the HTTP
Authorization: Basic authentication header.

NP: use of this per-action cachemgr_passwd is not secure and should be
avoided. Stronger security can be gained via http_access with regular
proxy_auth and other ACLs.