Add links to maxpsiposint, maxpsinegint, maxpsifracnum, and
maxpsifracdenom in the description of the function psi so the user of
the html doc can find them easily.
For each variable maxpsiposint, maxpsinegint, maxpsifracnum, and
maxpsifracdenom, use m4_math to make it a bit nicer to read in html,
and also add some examples on what impact these variables have on
computing psi[n](x).
For example, maxpsifracnum and maxpsifracdenom only affect psi[0](x).
But maxipsiposint and maxpsinegint affect all orders n.
specfun.lisp indirectly defines `$maxpsiposint`, `$maxpsinegint`,
`$maxpsifracnum`, and `$maxpsifracdenom` by setting the symbol-value
of these symbols. Since they're user-visible variables with entries
in the user manual, let's use `defmvar` to define them in
globals.lisp, with the appropriate initial values.
This also means `reset()` will reset the value to the default. This
didn't happen before, but I think it should.
Then we can remove the declare-top special for these from specfn.lisp.
`*last*` isn't a special var any more, so rename it to `sf-last` to
make it clear it's not special. Could have used just `last`, but
that's too simple and makes grepping for uses of this harder.
`sf-last` makes it unique. ("SF" for "specfun").
