MFC r31956[12]: Teach primes(6) to enumerate primes up to 2^64 - 1.

Approved by: re (delphij)
Relnotes: primes(6) now enumerates primes beyond 3825123056546413050,

up to a new limit of 2^64 - 1.

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_M .
M usr.bin/factor/factor.6
M usr.bin/primes/primes.c
M usr.bin/primes/primes.h
M usr.bin/primes/spsp.c