#1511 factor runs out of primes

[Barton Willis]

(%i21) factor(product(rat(x)-i,i,1,65));
Factor ran out of primes.

(%i22) build_info();
Maxima version: 5.15.0
Maxima build date: 17:36 4/20/2008
host type: i686-pc-mingw32
lisp-implementation-type: GNU Common Lisp (GCL)
lisp-implementation-version: GCL 2.6.8

[Dan Gildea]

Fixed in factor.lisp 1.18.

combine variable smallprimes (which went up to 61) with *small-primes*
(which goes up to 9973).

Ideally there would be no hard-coded limit.