Fix closeHandle bug, add setFileSize, make resize work on Windows (#21375)

* Add general purpose `setFileSize` (unexported for now).  Use to simplify
`memfiles.open` as well as make robust (via hard allocation, not merely
`ftruncate` address space allocation) on systems with `posix_fallocate`.

As part of this, fix a bad `closeHandle` return check bug on Windows and
add `MemFile.resize` for Windows now that setFileSize makes that easier.

* Adapt existing test to exercise newly portable `MemFile.resize`.

* Since Apple has never provided `posix_fallocate`, provide a fallback.
This is presently written in terms of `ftruncate`, but it can be
improved to use `F_PREALLOCATE` instead, as mentioned in a comment.

tests/stdlib/tmemfiles2.nim

@@ -15,8 +15,9 @@ var
 
 if fileExists(fn): removeFile(fn)
 
-# Create a new file, data all zeros
-mm = memfiles.open(fn, mode = fmReadWrite, newFileSize = 20)
+# Create a new file, data all zeros, starting at size 10
+mm = memfiles.open(fn, mode = fmReadWrite, newFileSize = 10, allowRemap=true)
+mm.resize 20  # resize up to 20
 mm.close()
 
 # read, change