Update the http(s)_port directives protocol= parameter

Update the http(s)_port directives protocol= parameter

... to use AnyP::ProtocolVersion for internal storage instead of opaque
string text.

This both limits the possible parameter values to one of HTTP, HTTP/1.1,
HTTPS or HTTPS/1.1 and ensures that URLs generated from that protocol
parameter value are http:// and https:// URL.
Other values will cause Squid to abort.