Change license from GPL-2 to something more permissive

[andreasabel]

BNFC currently is licensed under GPL-2 which is a bit strict and excludes e.g. industrial users from using the sample grammars (and even the bnfc tool) in closed-source projects.

The following list serves to organize the process and can be extended with new information.

TODO:

• Discuss on mailing list.
• Decide on a new license: BSD-3-clause.
• Draft letter to developers with instructions how to give written consent (public key / signed sheet of paper?)
• Set response date for reply and email to developers.
  * Decide whom to send this email (even 1-patch contributors? --- some did not even leave their real name).
  * For git developers, https://github.com/BNFC/bnfc/blob/master/.mailmap might help.
  * Collect emails of older developers.
• Await responses and add them to the repository (for future reference).
• Change global license setting in github.
• Remove GPL-2 text from all files. (Consider whether to swap in the new text instead---I'd rather not.)
• Fix some policies for authorship/copyright notices in files (currently, some files have the original authors, newer authors are hardly represented).

[andreasabel]

The delimiters pragma and the CNF backend are GPL licensed and have been removed for the switch to BSD.

[andreasabel]

2.9.0 has been released under BSD 3-clause.