Fix GH-7896: Environment vars may be mangled on Windows

When bug 77574[1] has been fixed, the fix only catered to variables
retrieved via `getenv()` with a `$varname` passed, but neither

Fix GH-7896: Environment vars may be mangled on Windows

When bug 77574[1] has been fixed, the fix only catered to variables
retrieved via `getenv()` with a `$varname` passed, but neither to
`getenv()` without arguments nor to the general import of environment
variables into `$_ENV` and `$_SERVER`. We catch up on this by using
`GetEnvironmentStringsW()` in `_php_import_environment_variables()` and
converting the encoding to whatever had been chosen by the user.

[1] <https://bugs.php.net/bug.php?id=75574>

Closes GH-7928.

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