Add C function dup and dup2 posix to system/io (#15675)

* * Add handle to dup and dup2 posix as duplicate and duplicateTo in std/ioutils.
* Added small test & changelog entry

* Fixed import in tioutils
removed when isMainModule

* * Nest test inside block. Rename proc var -> let in captureStdout
* Renamed tmpfile to iotuils.txt
* Added block: # duplicate, duplicateTo
* Improved docstring
* Clean non-idiomatic code
* Added runnable examples

* rm 2 trailing space in expected output

* Made syntax prettier

* Runnable example: file in getTempDir()

* Tmp -> Temp

* Fixed runnableExamples on windows

tests/stdlib/tioutils.nim

@@ -0,0 +1,49 @@
+discard """
+  output: '''
+hello1
+hello1
+'''
+"""
+
+import std/ioutils
+import os
+from stdtest/specialpaths import buildDir
+
+block: # duplicate, duplicateTo
+  let tmpFileName = buildDir / "tioutils.txt"
+  template captureStdout(body) : untyped =
+    let stdoutFileno = stdout.getFileHandle()
+    # Duplicate stoudFileno
+    let stdout_dupfd = duplicate(stdoutFileno)
+    # Create a new file
+    # You can use append strategy if you'd like
+    let tmpFile: File = open(tmpFileName, fmWrite)
+    # Get the FileHandle (the file descriptor) of your file
+    let tmpFileFd: FileHandle = tmpFile.getFileHandle()
+    # dup2 tmpFileFd to stdoutFileno -> writing to stdoutFileno now writes to tmpFile
+    duplicateTo(tmpFileFd, stdoutFileno)
+    body
+    # Force flush
+    tmpFile.flushFile()
+    # Close tmp
+    tmpFile.close()
+    # Read tmp
+    let ret = readFile(tmpFileName)
+    # Restore stdout
+    duplicateTo(stdout_dupfd, stdoutFileno)
+    ret
+
+  proc duplicateStdout() =
+    var msg = "hello"
+    echo msg & "1"
+
+    let s = captureStdout:
+      echo msg & "2"
+
+    doAssert s == "hello2\n"
+
+  discard tryRemoveFile(tmpFileName)
+  duplicateStdout()
+  # Check it works twice
+  duplicateStdout()
+  doAssert tryRemoveFile(tmpFileName)