Add reducefactors to find primes from many composites in stage1 GPU code

Try to determine the prime factors from an overlapping set of composite factors

(these are heavily edited for brevity)

Before

echo "101^2*1013*1000003*1000033*1000037*1000039*1000081^2*(2^89-1)^2" | ./ecm -gpu -gpucurves 1024 -sigma 3:1000 10000
Using B1=10000, B2=1873422, sigma=3:1000-3:2023 (1024 curves)
GPU: factor 10335659184464100091253526654407 found in Step 1 with curve 0 (-sigma 3:1000)
GPU: factor 10334016010907 found in Step 1 with curve 1 (-sigma 3:1001)
GPU: factor 10335700525736575955950356673123 found in Step 1 with curve 2 (-sigma 3:1002)
GPU: factor 10336072609839250164662144750599 found in Step 1 with curve 3 (-sigma 3:1003)
...
Computing 1024 Step 1 took 51ms of CPU time / 1691ms of GPU time
********** Factor found in step 1: 10335721196372813888298771682481
********** Factor found in step 1: 1000033
********** Factor found in step 1: 1000037
Found prime factor of 7 digits: 1000037
Composite cofactor (((101^2*1013*1000003*1000033*1000037*1000039*1000081^2*(2^89-1)^2)/10335721196372813888298771682481)/1000033)/1000037 has 54 digits

After

$ echo "101^2*1013*1000003*1000033*1000037*1000039*1000081^2*(2^89-1)^2" | ./ecm -gpu -gpucurves 1024 -sigma 3:1000 10000
Using B1=10000, B2=1873422, sigma=3:1000-3:2023 (1024 curves)
GPU: factor 10335659184464100091253526654407 found in Step 1 with curve 0 (-sigma 3:1000)
GPU: factor 10334016010907 found in Step 1 with curve 1 (-sigma 3:1001)
GPU: factor 10335700525736575955950356673123 found in Step 1 with curve 2 (-sigma 3:1002)
...
GPU: factor 10334367368536438055024419 found in Step 1 with curve 1021 (-sigma 3:2021)
GPU: factor 10335659184464100091253526654407 found in Step 1 with curve 1022 (-sigma 3:2022)
GPU: factor 10335721196372813888298771682481 found in Step 1 with curve 1023 (-sigma 3:2023)
Computing 1024 Step 1 took 31ms of CPU time / 1744ms of GPU time
********** Factor found in step 1: 1000003
********** Factor found in step 1: 1000033
********** Factor found in step 1: 1000037
********** Factor found in step 1: 1000039
********** Factor found in step 1: 10333613
********** Factor found in step 1: 1000162006561
Composite cofactor ((((((101^2*1013*1000003*1000033*1000037*1000039*1000081^2*(2^89-1)^2)/1000003)/1000033)/1000037)/1000039)/10333613)/1000162006561 has 54 digits

Note that 101^2*1013 appears in all found factors so it appears as a composite factor found (10333613) the same with 1000081^2 appearing as 1000162006561