Improve OS detection.

Replace `:haiku`, `:mezzano`, and `:genera` with `:os-` prefixed
alternatives.  This fixes a problem on SBCL where we were
inadvertently *removing* the `:haiku` feature.

Treat `:haiku` as a unix variant (fix `os-unix-p`).
9 jobs for !147 with iss37 in 39 minutes and 15 seconds (queued for 84 minutes and 43 seconds)
latest detached
Status Name Job ID Coverage
  Build
passed Build ASDF #11816

00:00:42

passed Build docs #11817

00:02:45

 
  Regression
passed ABCL 1.7.1 regression tests #11819

00:35:44

failed ABCL regression tests #11818
allowed to fail

00:31:53

passed CCL regression tests #11820

00:06:34

passed CLISP regression tests #11821

00:05:32

passed CMUCL regression tests #11822

00:06:36

passed ECL regression tests #11823

00:07:03

passed SBCL regression tests #11824

00:04:55

 
Name Stage Failure
failed
ABCL regression tests Regression
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