Minor updates to the OAEP code.

We now check that only one encoding method may be given. In the error
case we make sure that a released OAEP label variable is set to NULL.
As a failsafe feature we use gpg_err_code_from_syserror all over in
pubkey.c; this has the advantage that a misbehaving gcry_free function
which does not set ERRNO now returns an error code in all cases.