Travis has been super-slow lately (> 6 hours per build). Cirrus is much
faster: about 20 minutes. Cirrus also has slightly better test
coverage, mainly because it doesn't use SECCOMP.
Also,
* Fix the Redox CI build. The old Travis configuration didn't actually
build for Redox, so we never noticed that Redox can't be built with a
stable compiler. Thanks to @coolreader18 for finding this.
* Disable the udp_offload tests on cross-tested platforms. These tests
are failing with ENOPROTOOPT in Cirrus-CI. I suspect it's due to a
lack of support in QEMU. These tests were skipped on Travis because
its kernel was too old.
* Fix require_kernel_version on Cirrus-CI. Cirrus reports the Linux
kernel version as 4.19.112+, which the semver crate can't handle.
* Fix test_setfsuid on Cirrus. When run on Cirrus, it seems like the
file in /tmp gets deleted as soon as it's closed. Probably an
overzealous temporary file cleaner. Use /var/tmp, because no
temporary file cleaner should run in there.
* Skip mount tests on Cirrus. They fail for an unknown reason.
Issue #1351
* Skip the AF_ALG tests on Cirrus-CI
Issue #1352
