Rework pkey_dsa_sign() and pkey_dsa_verify()

Rework pkey_dsa_sign() and pkey_dsa_verify()

Since DSA_sign() and DSA_verify() ignore their type argument, don't bother
to determine it here. Check all size_t for overflow before passing them as
int arguments. Follow OpenSSL and add a check to see if the tbs blob's
length matches the one of the md, in case it is set on the EVP_PKEY_CTX.
Fix return value check of DSA_sign().

ok jsing