Fix non-exported `memfiles.setFileSize` to be able to shrink files on posix via `memfiles.resize` (#23717)

Fix non-exported `setFileSize` to take optional `oldSize` to (on posix)
shrink differently than it grows (`ftruncate` not `posix_fallocate`)
since it makes sense to assume the higher address space has already been
allocated there and include the old file size in the `proc resize` call.
Also, do not even try `setFileSize` in the first place unless the `open`
itself works by moving the call into the `if newFileSize != -1` branch.

Just cosmetics, also improve some old 2011 comments, note a logic diff
for callers using both `mappedSize` & `newFileSize` from windows branch
in case someone wants to fix that & simplify code formatting a little.

0 files changed, 0 insertions, 0 deletions