The default implementation of DSA_METHOD has an interdependence on the

dsa_mod_exp() and bn_mod_exp() handlers from dsa_do_verify() and
dsa_sign_setup(). When another DSA_METHOD implementation does not define
these lower-level handlers, it becomes impossible to do a fallback to
software on errors using a simple DSA_OpenSSL()->fn(key).

This change allows the default DSA_METHOD to function in such circumstances
by only using dsa_mod_exp() and bn_mod_exp() handlers if they exist,
otherwise using BIGNUM implementations directly (which is what those
handlers did before this change). There should be no noticable difference
for the software case, or indeed any custom case that didn't already
segfault, except perhaps that there is now one less level of indirection in
all cases.

PR: 507