Add project-wide license to Debian copyright file.

Unless otherwise specified, the whole project is considered
to be under the same terms as all non-borrowed code in
spectrwm.c. Quoting the wiki:

> spectrwm is ISC licensed software; we prefer code that
> comes with a simple license like the OpenBSD
> license.template, and all the spectrwm-specific code is
> licensed accordingly.
Small fixes to the Italian man page.
Update Debian changelog.