Add factor.

I was reading http://www.muppetlabs.com/~breadbox/txt/rsa.html and it mentioned
"factor" and I noticed it was in coreutils. I'm not sure why it's in coreutils,
but it's pretty trivial, so...
factor: catch integer overflow.

Now factor 9223372036854775783 (largest positive 64 bit signed prime) takes a
couple minutes but gives the right answer.