Update Http::ProtocolVersion() to initializer functions

Update Http::ProtocolVersion() to initializer functions

The Http::ProtocolVersion(*) does not work sufficiently well as a class
hierarchy.

Convert Http::ProtocolVersion to two functions:

* Http::ProtocolVersion() providing the default Squid HTTP version
level, and

* Http::ProtocolVersion(unsigned, unsigned) providing the HTTP version
details for the given level.

NP: using two overloaded functions instead of one with default
parameter values because with HTTP/0.x and HTTP/2.x we cannot safely
default just the minor value. ie. using two functions prevents
mistakenly using HTTP/2.1, HTTP/0.1 or HTTP/1.0 if the second
parameter is omitted.

All variables must now be of type AnyP::ProtocolVersion, and should be
constructed from an appropriate Foo::ProtocolVersion() function.