Fix prime test for 2 and lower and add check command to mpicalc.

* cipher/primegen.c (check_prime): Return true for the small primes.
(_gcry_prime_check): Return correct values for 2 and lower numbers.

* src/mpicalc.c (do_primecheck): New.
(main): Add command 'P'.
(main): Allow for larger input data.
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(cherry picked from commit 5c906e2cdb14e93fb4915fdc69c7353a5fa35709)