Instead of using the floating point square root, use an integer version

Instead of using the floating point square root, use an integer version
of the Newton method from ping.c.  Fixes a rounding issue that caused
failure to factor numbers close to 2^64, e.g. 18446744030759878681.
While there, fix an off by one error that caused 4295360521 to be
reported as a prime.  Issues reported by Paul Stoeber and Michael Bozon.

ok tedu, deraadt