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.