Implement REUSE 3.0 with SPDX identifiers

Implement REUSE 3.0 with SPDX identifiers

Implement the full REUSE spec [1] to help with copyright and licensing
audits and compliance. This will reduce a lot of manual work for the
licensing audit required in Debian on each update and also allow using
automated tools.

For files that lacked copyright and licensing information, I used the
following guidelines. If a clear author could be determined from the git
history use it, otherwise use 'EfficiOS Inc.'. For code use
'GPL-2.0-only OR LGPL-2.1-only' unless otherwise stated, for
documentation 'CC-BY-SA-4.0' and for data files 'CC0-1.0'.

Freeform text files were converted to Markdown to allow licensing
comments.

Running the reuse tool on the repo is now succesful:

  $ reuse lint

  # SUMMARY

  * Bad licenses: 0
  * Deprecated licenses: 0
  * Licenses without file extension: 0
  * Missing licenses: 0
  * Unused licenses: 0
  * Used licenses: CC0-1.0, GPL-2.0-only, CC-BY-SA-4.0, MIT, LGPL-2.1-only
  * Read errors: 0
  * files with copyright information: 358 / 358
  * files with license information: 358 / 358

  Congratulations! Your project is compliant with version 3.0 of the REUSE Specification :-)

[1] https://reuse.software/tutorial/

Change-Id: I1755cab24a6fcec7a6c9a2136891418203ec34b8
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>