to replace the old --level option. Added sypport for -a symmetric,
which generates random keys for use as a shared secret.
(check_file): Allow file to be NULL.
(process_private): Hex encode symmetric keys.
(main): Use the value of the --bit-length option. New logic to
handle generation of symmetric keys.
(dsa_generate_key): Take bit size as input, not a NIST security
levell.

Rev: src/lsh-keygen.c:1.26