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").
