Merge remote-tracking branch 'remotes/elmarco/tags/slirp-pull-request' into staging

slirp: clarify license of slirp as BSD-3

# gpg: Signature made Fri 22 Mar 2019 19:16:50 GMT
# gpg:                using RSA key DAE8E10975969CE5
# gpg: Good signature from "Marc-André Lureau <marcandre.lureau@redhat.com>" [full]
# gpg:                 aka "Marc-André Lureau <marcandre.lureau@gmail.com>" [full]
# Primary key fingerprint: 87A9 BD93 3F87 C606 D276  F62D DAE8 E109 7596 9CE5

* remotes/elmarco/tags/slirp-pull-request:
  slirp: is not maintained by Kelly Price for a long time
  slirp: remove reference to COPYRIGHT file
  slirp: clarify license of slirp files using SPDX: implicit via unstated
  slirp: clarify license of slirp files using SPDX: implicit via COPYRIGHT
  slirp: clarify license of slirp files using SPDX: explicit MIT
  slirp: clarify license of slirp files using SPDX: explicit BSD
  slirp: relicense GPL files to BSD-3
  slirp: update COPYRIGHT to use full 3-Clause BSD License

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>