more enhancements for #11618 (#11976)

* finish the Windows IO layer changes; refs #11618
* added system.getOsFileHandle which is less error-prone on Windows
* make tests green again

tests/system/techo_unicode.nim

@@ -0,0 +1,27 @@
+discard """
+  output: '''ÄhmÖÜ
+abasdfdsmÄhmaИ
+Иnastystring
+A你好
+ИnastystringA你好
+ÖÜhmabasdfdsmÄhmaИ'''
+  disabled: "posix"
+  joinable: "false"
+"""
+
+import winlean
+
+echo "ÄhmÖÜ"
+echo "abasdfdsmÄhmaИ"
+echo "И\0nasty\0\0\0\0string\0"
+echo "A你好"
+
+write stdout, "И\0nasty\0\0\0\0string\0"
+writeLine stdout, "A你好"
+stdout.flushFile()
+
+let handle = getOsFileHandle(stdout)
+var a = "ÖÜhmabasdfdsmÄhmaИ"
+var ac = 0'i32
+discard writeFile(handle, addr a[0], int32(len(a)), addr ac, nil)
+stdout.flushFile()