Add dummy terms in a way that minimises the number of new gap sizes.

Add extra dummy terms to bring the total number of terms up to a multiple of
SWIZZLE. This should allow sieve64() to be simplified a little.

When b=2 and gmul=1, use modular addition instead of modular multiplication
to build the table of powers.

Allow the sieve file to be in the NewPGen format as used by LLR.

When using SSE2 (i.e. 64-bit instead of 80-bit doubles) for the floating
point mulmod operations (x86-64), conditional moves start to become faster
than branches as p increases beyond 1000e12. Choose which method to use based
on the size of pmax.

Avoid referencing G[][] and N[][] arrays after initialization, so that they
can be freed before sieving starts.

Integrate the SMALL_P code into the standard executable.
