GH367: Fix dsa keygen for too-short seed

If the seed value for dsa key generation is too short (< qsize),
return an error. Also update the documentation.

Signed-off-by: Rich Salz <rsalz@akamai.com>
Reviewed-by: Emilia Käsper <emilia@openssl.org>
(cherry picked from commit f00a10b8)