dc(1) exponent limits

[Lyndon Nerenberg <lyndon@orthanc.ca>]

While running some silly benchmarks I discovered dc's '^' operator
limits exponents to '9999'.  This seems arbitrary, perhaps a leftover
safety measure to keep things from eating all the CPU for days on
end on a slow machine?  I upped the limit to 99999 and the test
expression ran fine on a Pi4:

/n/dump/2019/1215.2/sys/src/cmd/dc.c:328,334 - dc.c:328,334
  				neg++;
  				chsign(arg1);
  			}
- 			if(length(arg1)>=3) {
+ 			if(length(arg1)>=4) {
  				error("exp too big\n");
  			}
  			savk = sunputc(arg2);

If you're feeling bored and apply the above patch, consider running
this mini-bench and mailing the output directly to me:

echo -n `{cat /dev/cputype}^' ** ' ; echo 652342 52342 '^' 34232342 / p q | time dc >/dev/null

It will take a while to run (50 minutes on the Pi4).

--lyndon