io: fix actual and latent bugs in `io.die` and `io.warn`.

* specs/spec_helper.lua.in (luaproc): Factor out of
`tabulate_output`.
(tabulate_output): Adjust.
* specs/io_spec.lua (die, warn): Add new behaviour examples.
* lib/std/io.lua (warn): Adjust to work properly with new specs.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>