Work around a documentation toolchain problem by replacing the "AIX-fixlevels"

Work around a documentation toolchain problem by replacing the "AIX-fixlevels"
table with a <variablelist> carrying the same information.  Previously the
9.0 documentation was failing to build as a US-size PDF file.  It's quite
obscure what the real problem is or why this avoids it, but we need a hack
now so we can build docs for beta4.

In passing do a bit of editing in the AIX installation docs, in particular
remove a long-obsolete claim that the regression tests are likely to fail.