This required updating to Ubuntu Xenial for some of the cross compilers, but Travis doesn't support Xenial builders, so this instead converts Travis to use the already-existing "ddist.sh" script for building via Docker.
Also fixed a bug with the signals test, which didn't properly exercise
Tini: rather than check that Tini was properly exiting with 128 +
signal, it raced against Tini and was only successful if Tini didn't get
the change to spawn a subprocess!
Also adding a test in run_build.sh to ensure that the pre-processed
license is always up to date with the actual license file.
To recreate: `xxd -i LICENSE > src/license.h`
Tianon Gravi
Thomas Orozco