Function to truncate a file

[Paul K <paul@...>]

I'd like to come back to Daurnimator's suggestion to add a file
truncation function to Lua
(http://lua-users.org/lists/lua-l/2011-12/msg00111.html).

I know Luiz responded "truncating to zero length is the same as
opening for writing, except for dates", but it's beneficial in many
cases to write to the same file, as it preserves its attributes,
metadata, permissions, etc.

Paul.