Force the OIDs read from input files to be interpreted as big-endian integers.
Leaving them as little-endian results in invalid values in params, eventually
leading to a crash.

Previous code allocated an array on the stack of mlen bytes, but
it should be possible to also sign heap-space messages. By relying
on the fact that sm and m fit the message + signature, we move
the message so that 4*n bytes of prefix can be added.