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

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