Switch all builds from Travis to Cirrus

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

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