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.