Check if large factor is prime before applying Pollard's algorithm;
fixes "factor 2147483647111311". Increase base if p-1 algorithm
reaches 1; fixes "factor 99999999999991".
Testcases from David A Bagley <bagleyd@tux.org>.
Fixes from Joseph Myers <jsm@NetBSD.org>.
Problem rediscovered by an attempt to factor my phone number.
A few other incidental fixes: correct a couple of factually incorrect
comments; use ident string macros; move from 4-clause to 3-clause
BSD licence (University of California copyright).
Obtained from: NetBSD
38281fea3e