Version 1.0.5: Added a PRP test of each candidate surviving the sieve.

Version 1.0.6: Re-ordered the PRP test to allow re-use of an intermediate value.
Version 1.0.7: Compute 2^2^n as 4^2^(n-1) when PRP=1. (a little faster).
               Delay computing M(1) until after the PRP test.

Version 1.0.8: Use bits instead of bytes for the sieve bitmap.
	       Use alarm() to report progress every 15 seconds.

    Source: geocities.com/g_w_reynolds/gfn

               ( geocities.com/g_w_reynolds)