Switching to SPDX license tags

[Thomas Klausner <wiz%NetBSD.org@localhost>]

Hi!

SPDX has a list of license names:

https://spdx.org/licenses/

This includes 'Identifiers' which correspond in idea to the license
names we have in pkgsrc, and markers if they're FSF/OSI approved.

I think it'd be useful to convert our licenses (LICENSE) to these
standardized identifiers. Perhaps url2pkg could even set it
automatically by looking for SPDX-License tags in the code,
e.g. these:

// SPDX-License-Identifier: MIT

For the migration, I'm thinking of these steps:

- add the licenses corresponding to the new SPDX names to
  pkgsrc/licenses

- add the SPDX names of the currently 'default accepted' licenses to
  the new default accepted lists in pkgsrc/mk/license.mk and
  pkg_install

- run a regex over pkgsrc's LICENSE lines replacing the current names
  with the SPDX names

- Clean up: remove the old license files from mk/licenses; at some
  later point (a year later?) remove the old license names from the
  default acceptable lists

We'll have to manually fix entries; e.g. currently we use gnu-gpl-v2
for both 'GPL v2' and 'GPL v2 or later' while SPDX has GPL-2.0-only
and GPL-2.0-or-later, so help will be needed at that point.

As for the timeframe, I expect to implement this shortly after the
next branch, so early April 2023.

Comments, suggestions?

Cheers,
 Thomas