lib/licenses.nix: convert to spdx.org shortnames

Done where possible, replacing links to point to spdx.org (probably stabler).
Also make libtiff use its dedicated license.
lib/licenses.nix: improve sortedness
lib/licenses.nix: allow choosing license by its shortName

Example: license = licenses."GPL-2.0+";
This comes with some performance cost during eval of licenses.nix,
but that's probably negligible.
xlsfonts, xfontsel: fix meta typo
doc/meta.xml: fix github links, clarify licensing section
Revert "lib/licenses.nix: allow choosing license by its shortName"

There's no consensus to adopt that (yet).
This reverts commit 229e5c41df8db960de9d57b247107b6239a60383.
Merge #3408: licenses: prefer spdx.org, improve docs

The patch to allow using license shortnames as attributes
was not included (yet).

Conflicts (auto-solved):
	pkgs/development/libraries/libtiff/default.nix