truncating a file

[Daurnimator <quae@...>]

I've recently run into the problem of trying to truncate a file in
lua... which doesn't seem to be possible
Not only is it not covered by the standard library; its also not in lfs :(

So i'd like to humbly request we can add one more little
non-covered-by-ansi-C feature to liolib.c
f_truncate

which uses _chsize_s on windows or ftruncate on POSIX
for platforms with neither; we should have a similar situation to io.popen

*hoping this can make it into 5.2 even though we're up to rc*
D