* And simplify callers of get_actual_encoding().
* See [Feature #18949].
* See https://github.com/ruby/ruby/pull/6322#issuecomment-1242758474

See https://bugs.ruby-lang.org/issues/18338

This reverts commit a698181021c11711dc56b74dacbdebc2895b5384.

It failed on macos for a unknown problem.
```
    1) Error:
  TestM17N#test_object_inspect_external:
  Encoding::ConverterNotFoundError: code converter not found (US-ASCII to UTF-16BE)
      /Users/runner/work/ruby/ruby/src/test/ruby/test_m17n.rb:312:in `encode'
      /Users/runner/work/ruby/ruby/src/test/ruby/test_m17n.rb:312:in `inspect'
      /Users/runner/work/ruby/ruby/src/test/ruby/test_m17n.rb:315:in `inspect'
      /Users/runner/work/ruby/ruby/src/test/ruby/test_m17n.rb:315:in `<main>'
      /Users/runner/work/ruby/ruby/src/test/ruby/test_m17n.rb:299:in `test_object_inspect_external'
```
https://github.com/ruby/ruby/runs/4207871418?check_suite_focus=true

The test uses `Encoding.default_external = Encoding::UTF_16BE`, which
may add a wrongly UTF_16BE-encoded path to $LOADED_FEATURES (depending
on the order of tests). Unfortunately this breaks another test:

http://ci.rvm.jp/results/trunk-test@ruby-sky1/3711615
```
/tmp/ruby/v3/src/trunk-test/test/io/console/test_io_console.rb:11:in `===': incompatible encoding regexp match (US-ASCII regexp with UTF-16BE string) (Encoding::CompatibilityError)
```

According to @naruse-san, we don't pay effort to such a case, so this
change just avoids the issue by running the test in question under
another process.

Co-Authored-By: Koichi Sasada <ko1@atdot.net>

* 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.