Remove references to SunOS 4

Remove references to SunOS 4

We already removed support for Solaris versions before Solaris 8, in
commit e4c2810f ("Remove support for Solaris pre-8"), but there are
still some references to SunOS (meaning SunOS 4) in the tree. This is
even older than Solaris (aka SunOS 5), so get rid of these.

This commit removes most references to SunOS 4 regarding platform
support, and a few comments. This also removes a few comments that
were just wrong or nonsensical (e.g. CMAPPED in afs.h is used by
other platforms; some comments in platform-specific osi_file.c files
referenced SunOS for some reason).

Change-Id: I0dd3176c582409176fd898f9c9539fbd833ea789
Reviewed-on: https://gerrit.openafs.org/13506
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>