VS: Refactor generator toolset parsing

We parse `CMAKE_GENERATOR_TOOLSET` values of the forms:

* `toolset`
* `toolset,host=x64`
* `host=x64`

Generalize the parsing to support the forms:

* `toolset`
* `toolset[,key=value]*`
* `key=value[,key=value]*`

Disallow duplicate keys.  Require all but the first field to
be of `key=value` form.