crypto: dh - implement private key generation primitive for ffdheXYZ(dh)

The support for NVME in-band authentication currently in the works ([1])
needs to generate ephemeral DH keys for use with the RFC 7919 safe-prime
FFDHE groups.

In analogy to ECDH and its ecc_gen_privkey(), implement a
dh_safe_prime_gen_privkey() and invoke it from the ffdheXYZ(dh) templates'
common ->set_secret(), i.e. dh_safe_prime_set_secret(), in case the input
->key_size is zero.

As the RFC 7919 FFDHE groups are classified as approved safe-prime groups
by SP800-56Arev3, it's worthwhile to make the new
dh_safe_prime_gen_privkey() to follow the approach specified in
SP800-56Arev3, sec. 5.6.1.1.3 ("Key-Pair Generation Using Extra Random
Bits") in order to achieve conformance.

SP800-56Arev3 specifies a lower as well as an upper bound on the generated
key's length:
- it must be >= two times the maximum supported security strength of
  the group in question and
- it must be <= the length of the domain parameter Q.

For any safe-prim...