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!
This adds 1k of weight to the resulting binary, which is reasonable
(it's less than 5% for the smaller non-static binary), but alleviates
legitimate user concern that the license requires being included when
Tini is redistributed.
The GPG signing subkey and passphrase are respectively provided through
a Travis encrypted file and a Travis encrypted environment variable.
Signing is only done if there is a signing key present when the build is
complete (so as to not fail when e.g. building a PR that doesn't have
encrypted files available).
Using the child subreaper mechanism, we can actually run
tests inside the CI environment without depending on Docker.
While this does not replace the existing tests, it allows
at least some functional coverage within CI.