Add support for sockaddr_un on Windows. (#6513)

* Windows: Fix warning about undefined if_indextoname()

* Windows: Fix UNIXSocket on MINGW and make .pair more reliable

* Windows: Use nonblock=true for read tests with scheduler

* Windows: Move socket detection from File.socket? to File.stat

Add S_IFSOCK to Windows and interpret reparse points accordingly.
Enable tests that work now.

* Windows: Use wide-char functions to UNIXSocket

This fixes behaviour with non-ASCII characters.
It also fixes deletion of temporary UNIXSocket.pair files.

* Windows: Add UNIXSocket tests for specifics of Windows impl.

* Windows: fix VC build due to missing _snwprintf

Avoid usage of _snwprintf, since it fails linking ruby.dll like so:

  linking shared-library x64-vcruntime140-ruby320.dll
  x64-vcruntime140-ruby320.def : error LNK2001: unresolved external symbol snwprintf
  x64-vcruntime140-ruby320.def : error LNK2001: unresolved external symbol vsnwprintf_l

whereas linking miniruby.exe succeeds.

This patch uses snprintf on the UTF-8 string instead.

Also remove branch GetWindowsDirectoryW, since it doesn't work.

* Windows: Fix dangling symlink test failures

Co-authored-by: Lars Kanis <[email protected]>

test/fileutils/test_fileutils.rb

@@ -472,10 +472,14 @@ def test_cp_r_dev
   else
     def test_cp_r_socket
       pend "Skipping socket test on JRuby" if RUBY_ENGINE == 'jruby'
+
       Dir.mkdir('tmp/cpr_src')
       UNIXServer.new('tmp/cpr_src/socket').close
       cp_r 'tmp/cpr_src', 'tmp/cpr_dest'
       assert_equal(true, File.socket?('tmp/cpr_dest/socket'))
+    rescue Errno::EINVAL => error
+      # On some platforms (windows) sockets cannot be copied by FileUtils.
+      omit error.message
     end if defined?(UNIXServer)
   end