`rb_io_puts` should not write zero length strings. (#7806)

	---
	 io.c                  | 40 ++++++++++++++++++++++++----------------
	 test/fiber/test_io.rb | 28 ++++++++++++++++++++++++++++
	 2 files changed, 52 insertions(+), 16 deletions(-)

* 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 <kanis@comcard.de>

There were two issues:

1. When an IO object is waiting for writablility only (as in test_tcp_accept) the selected hash is empty.
   Therefore selected[fiber] returns nil but needs to default to 0 in order to be or'ed with IO::WRITABLE.

2. When an IO object is waiting for read- or writability (as in test_tcp_connect), but only one of these
   two events arrive, the Fiber and IO object need to be removed from the other `@readable` or `@writable` list.