package/slirp: security bump to version 4.3.1

package/slirp: security bump to version 4.3.1

- Use an up to date fork (spice slirp is archived and has not been
  updated since 2012)
- Add COPYRIGHT as the license file
- BSD-4-Clause has been replaced by BSD-3-Clause since
  https://gitlab.freedesktop.org/slirp/libslirp/-/commit/3bac39137a652b24b89d5b9e2a39600619fbe1d3
  https://gitlab.freedesktop.org/slirp/libslirp/-/commit/f9f6e69c4e1d9a43af30bfe791b31789ffa04954
- Add hash file
- Switch to meson-package
- Fix multiple security vulnerabilities: CVE-2014-3640, CVE-2017-11434,
  CVE-2019-6778, CVE-2019-9824, CVE-2019-14378 and CVE-2020-10756

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>