Portable format specifiers fix (%lu using EMSGN instead of EMSGU).

In code before portable-format-specifiers task, there was a bug:
The only case of a `"%lu"` format string was using `EMSGN` instead of `EMSGU`, which is incorrect since `EMSGN` will cast its argument to `int64_t` (previously `long`).

In fact, `EMSGU` macro was defined as the unsigned counterpart of `EMSGN` (thus taking a `long_u` instead of a `long`), and invoking `emsgu` instead of `emsgn`.

But, `emsgu` was not defined anywhere, which didn't result in an error, because EMSGU was in fact not used, the only case of a `"%lu"' erroneously using `EMSGN`, as explained above.

I missed this all when doing the main task.
This PR corrects that all.