Remove KLH10S_CENV_CPU_ and KLH10S_CENV_SYS_ and CENV_CPUF_BIGEND and
CENV_CPUF_LILEND since they are all already detected and defined by the
configure script.

because it is always detected by configure, so the OS-specific cases
do never apply.

by just using configure to find out which headers exist, rather than
guessing based on dubious system guesses.

I was hoping to remove more of them: at least CENV_SYS_BSD for example which
would allow removing several more.
Unfortunately CENV_SYS_BSD is used in tape control behaviour checks.
There is no way of checking those with configure. Worse, how well tape
worked on various Unixen when the tape code was written may be totally
different in later versions.
For signal handling I could at least remove CENV_SYS_BSD by implementing
the suggestion to use sigvec(2) (now deprecated). Falling back to plain
V7-style signal(2) is probably not good enough, so that is an error case
now. Fortunately that is totally unneeded on any modern Unix.