[doc] [license] Improve licensing situation.

Should hopefully close #266 , thanks to Julien Puydt and Karl Palmskog
for helping with this.

- Fix LICENSE and opam file discrepancies
- Update LICENSE from `debian/copyright` work by Julien
- Fix file headers
- Move JS files to its own util files (can be also removed in favor of
  `base` these days)

CHANGES.md

@@ -1,3 +1,8 @@
+## unreleased:
+
+ - [general] Make licensing clearer (@ejgallego @palmskog
+             @SnarkBoojum, # , closes #266)
+
 ## Version 0.18.1:
 
  - [serlib] Fix a few 8.18 piercings (!) (@ejgallego, #357)

LICENSE

@@ -3,22 +3,20 @@ Upstream-Name: coq-serapi
 Upstream-Contact: Emilio J. Gallego Arias <e@x80.org>
 Source: https://github.com/ejgallego/coq-serapi
 License: LGPL-2.1+
-Copyright: 2016-2022, MINES ParisTech / Inria
+Copyright: 2016-2023, MINES ParisTech / Inria / others
+Authors: Emilio J. Gallego Arias, Karl Palmskog, Clément Pit-Claudel, Kaiyu Yang
 
-Authors: Emilio J. Gallego Arias
+Files: serlib/* serapi/* sertop/*
+License: LGPL-2.1+
+Copyright: 2016-2023, MINES ParisTech / Inria / others
 
 Files: notes/*
 License: LGPL-2.1+ / CC-BY-SA 3.0
 
-Files: jscoq/*
-License: GPL-3+
-
 Files: tests/genarg/*
 Authors: Karl Palmskog
 Licence: Derived from many projects as test cases, falls into fair-use
 
 Comment: the intent is to be compatible with Coq's license. Note that
-the LGPL-2.1+ allows to treat the code as GPL-3+ , and we favor
-that. In some cases, like when linking with jsCoq's code or other code
-that is GPL-3 only, the resulting .js or binaries files will be GPL-3.
+the LGPL-2.1+ allows to treat the code as GPL-3+ , and we favor that.
 

coq-serapi.opam

@@ -3,7 +3,7 @@ maintainer:   "e@x80.org"
 homepage:     "https://github.com/ejgallego/coq-serapi"
 bug-reports:  "https://github.com/ejgallego/coq-serapi/issues"
 dev-repo:     "git+https://github.com/ejgallego/coq-serapi.git"
-license:      "GPL-3.0-or-later"
+license:      "LGPL-2.1-or-later"
 doc:          "https://ejgallego.github.io/coq-serapi/"
 
 synopsis:     "Serialization library and protocol for machine interaction with the Coq proof assistant"