REUSE requires the full license texts to exist in the LICENSES
directory. We have three licenses at the moment:
- BSD-2

-Clause-Patent for most of the code;
- ISC for the parts taken from iw;
- MIT for easylogging++.

Replace LICENSE with a symlink to the text in LICENSES. The text in
LICENSES is taken as is from the SPDX definition. It differs only in
whitespace, and in the copyright line itself. Since it's better to use
the literal text from the SPDX definition, move the copyright years to
the AUTHORS.md file. While we're at it, correct those years: the start
year is 2016 for some of the code, and the end year is 2020 now.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>