Let gcry_pk_decrypt in non-raw mode return a verbatim buffer.

The non-raw modes of gcry_pk_decrypt (i.e. pkcs1 or oaep un-padding)
are new and thus we can still change the semantics.

They now return a verbatim buffer and not anything which internally
has been interpreted as a signed integer. In raw mode we still stick
to the old semantics which is usually fine because it is mostly used
with pkcs#1 padding and that guarantees that the return value may
never be interpreted as a signed MPI or shorted due to block type used
as the second byte.