Rationalize the default httpd_accel_surrogate_id

Rationalize the default httpd_accel_surrogate_id

Prevents more posibilities for data leakage by making the default
surrogate ID based on visible_hostname (public FQDN for the proxy).

Now that Surrogate_Capability: header is sent by default in accelerator
environments it makes more sense to default it to a value fairly unique
or at least restricted to that local administrative domain.

When visible_hostname is setup correctly (either automatic or manual)
each stand-alone squid install should have a unique ID. Groups of proxies
sharing work for a domain should also be by default sharing an ID and
thus sharing the override behavior.

When visible_hostname is unavailable it will retain the old default of
'unset-id'.