midend_deserialise: accept an extra validation function.

midend_deserialise: accept an extra validation function.

This will let me do a 'conditional deserialisation' operation, in
which we fully decode the serialised data and then (assuming that gave
no errors) decide whether or not to actually install it based on some
arbitrary condition.

I don't think there's any possible use for the extra check function
_outside_ midend.c, so I've left the API for front ends as it is; the
externally visible midend_deserialise() doesn't have the new
parameter, and only midend_deserialise_internal() includes it.