Rework pkey_rsa_keygen()

Rework pkey_rsa_keygen()

As usual, make the function single exit. Initialize the pkey callback
pointer and the BN_GENCB on the stack at the top rather than relying
on the weird trans_cb() in evp_pkey_set_cb_translate() to do so.
Greatly simplify the control flow and add missing error checks.

ok jsing