Replace client 'version' with 'variant', 'type' and 'subtype'

[slook]

There is ambiguity about the purpose of the "major version" and "minor version" values as to what they are supposed to represent. It needs to be made clearer that project maintainers must avoid impersonating other client implementations.

• Changed: client_version -> client_variant because this isn't the "version of the user's client software"
• Changed: major_version -> client_type because this is expected to be constant as to a particular project
• Changed: minor_version -> client_subtype because this is unrelated to the user's client software version
• Changed: "%version%" -> "%variant%" because it is ambiguous with "%sversion%" could become "%version%"

Other possible alternatives considered for "major" -> "type" | "identifier" | "kind" | "family" | "class" ...?
Other possible alternatives considered for "minor" -> "subtype" | "revision" | "generation" | "subclass" ...?