Merge remote-tracking branch 'remotes/gkurz/tags/9p-next-2019-12-12' into staging

- conversion of virtfs-proxy-helper from libcap to libcap-ng
- removal of libcap-dev from docker, travis and gitlab CI
- removal of deprecate "-virtfs_synth" option

# gpg: Signature made Thu 12 Dec 2019 19:55:53 GMT
# gpg:                using RSA key B4828BAF943140CEF2A3491071D4D5E5822F73D6
# gpg: Good signature from "Greg Kurz <groug@kaod.org>" [full]
# gpg:                 aka "Gregory Kurz <gregory.kurz@free.fr>" [full]
# gpg:                 aka "[jpeg image of size 3330]" [full]
# Primary key fingerprint: B482 8BAF 9431 40CE F2A3  4910 71D4 D5E5 822F 73D6

* remotes/gkurz/tags/9p-next-2019-12-12:
  virtfs: Remove the deprecated "-virtfs_synth" option
  travis.yml: Drop libcap-dev
  ci: Use libcap-ng
  docker: remove libcap development packages
  virtfs-proxy-helper: switch from libcap to libcap-ng

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