mark nsmb major obsolete

Systematize handling of removed drivers.

- Every driver that was removed and whose number hasn't already been
reused is now listed with a commented-out "obsolete" line.
- The format of these has been systematized. Future format changes can
probably be safely done with a script.
- This does not include a few cases of assignments that only lasted a
couple days, or stuff from before major reorgs. Some of these may
be included nonetheless, because there was a lot of ground to cover
and therefore not a lot of time to dig into history in detail.

Note that the obsolete listings do not mean the major numbers can
never be reused; that's up to portmasters and/or core. It does mean
that they won't be reused by accident, however, which in some cases
(depending on the driver, how widely used it was, its family of device
nodes, their default permissions, etc.) can be quite dangerous.

Note that some of the things now explicitly listed as obsolete are
really ancient history. My scan went back as far as when the majors
files were added. (But not before that.)

Remove compat_svr4 and compat_svr4_32, as discussed on tech-kern@ recently,
but also as discussed several times in the past.

branches: 1.2.4; 1.2.8; 1.2.32; 1.2.34;
Merge riastradh-drm2 to HEAD.

branches: 1.1.2; 1.1.4;
Add stub port for evbcf which has enough to do a distribution build.