apps/dsaparam.c generates code that is intended to be pasted or included

into an existing source file: the function is static, and the code
doesn't include dsa.h.  Match the generated C source style of dsaparam.

Adjust apps/dhparam.c to match, and rename the BIGNUMs to their more
usual single-letter names.  Add an error return in the generated C source.

both: simplify the callback function

Signed-off-by: Beat Bolli <dev@drbeat.li>

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/6797)