* Windows: Read ENV names and values as UTF-8 encoded Strings

Implements issue #12650: fix https://bugs.ruby-lang.org/issues/12650

This also removes the special encoding for ENV['PATH'] and some
complexity in the code that is unnecessary now.

* Windows: Improve readablity of getenv() encoding

getenv() did use the expected codepage as an implicit parameter of the macro.
This is mis-leading since include/ruby/win32.h has a different definition.
Using the "cp" variable explicit (like the other function calls) makes it
more readable and consistent.

* Windows: Change external C-API macros getenv() and execv() to use UTF-8

They used to process and return strings with locale encoding,
but since all ruby-internal spawn and environment functions use UTF-8,
it makes sense to change the C-API equally.