stdlib/os: handle symlinks in copy/move functions (#16709)

* stdlib/os: handle symlinks in copy/move functions

- Added optional `options` argument to `copyFile`, `copyFileToDir`, and
  `copyFileWithPermissions`. By default, symlinks are followed (copy files
  symlinks point to).
- `copyDir` and `copyDirWithPermissions` copy symlinks as symlinks (instead of
  skipping them as it was before).
- `moveFile` and `moveDir` move symlinks as symlinks (instead of skipping them
  sometimes as it was before).
- Added optional `followSymlinks` argument to `setFilePermissions`.

See also: https://github.com/nim-lang/RFCs/issues/319

Co-authored-by: Timothee Cour <timothee.cour2@gmail.com>

* Address comments in #16709

Co-authored-by: Timothee Cour <timothee.cour2@gmail.com>

* Address comments in #16709 (second iteration)

Skip symlinks on Windows.

Co-authored-by: Timothee Cour <timothee.cour2@gmail.com>

tests/test_nimscript.nims

@@ -3,6 +3,8 @@
 
 {.warning[UnusedImport]: off.}
 
+from stdtest/specialpaths import buildDir
+
 import std/[
   # Core:
   bitops, typetraits, lenientops, macros, volatile,
@@ -87,3 +89,28 @@ block:
   try: doAssert false
   except Exception as e:
     discard
+
+block:  # cpDir, cpFile, dirExists, fileExists, mkDir, mvDir, mvFile, rmDir, rmFile
+  const dname = buildDir/"D20210121T175016"
+  const subDir = dname/"sub"
+  const subDir2 = dname/"sub2"
+  const fpath = subDir/"f"
+  const fpath2 = subDir/"f2"
+  const fpath3 = subDir2/"f"
+  mkDir(subDir)
+  writeFile(fpath, "some text")
+  cpFile(fpath, fpath2)
+  doAssert fileExists(fpath2)
+  rmFile(fpath2)
+  cpDir(subDir, subDir2)
+  doAssert fileExists(fpath3)
+  rmDir(subDir2)
+  mvFile(fpath, fpath2)
+  doAssert not fileExists(fpath)
+  doAssert fileExists(fpath2)
+  mvFile(fpath2, fpath)
+  mvDir(subDir, subDir2)
+  doAssert not dirExists(subDir)
+  doAssert dirExists(subDir2)
+  mvDir(subDir2, subDir)
+  rmDir(dname)