optparse: integrate with `io.die` and `io.warn`.

* specs/io_spec.yaml: Additional specs for what to do when
`opts.program` and/or `opts.line` are set.
* specs/optparse_spec.yaml (io.die, io.warn): New specs how to
behave when `io.die` or `io.warn` are called after a successful
`parser:parse` call.
* lib/std/io.lua (warn): Implement specified behaviour for new
`opts.line` and `opts.warn` specs.
* lib/std/optparse.lua: Add LDocs for new behaviour.
(parser:parse): Set a metatable on the returned opts table so
than `opts.program` etc. can be found by `io.die` and `io.warn`.
* NEWS: Update.

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