Remove long_u: put_bytes(): Refactor.

Remove all long_u instances due to put_bytes() function.

First, function signature is changed this way:
- nr  : long_u --> uintmax_t
    uintmax_t is chosen so that invocations can use any unsigned integer
    type (including size_t) without needing to cast.
- len : int    --> unsigned int
    This is to pass the size in bytes of the previous param, thus an
    unsigned int is enough. All invocations use positive integer
    literals, so change is safe without the need for casts.

Then, function implementation is adapted accordingly.

Last, all invocation points are refactored this way:
- Refactor types to minimize casts.
- Inline declarations (C99 style) in containing function.

All this changes were done with -Wconversion temporarily activated for
spell.c and undo.c, so that we can assert changes are type-safe and do
not introduce any warnings to that respect.
Remove long_u: Passing-by: undo.c: Add to -Wconversion.

Previous commit dropped many -Wconversion warnings in both spell.c and
undo.c. spell.c still has a lot of them (200+). But in undo.c, only a
handful of them remain. Take the chance to eliminate those, too, and add
undo.c to -Wconversion checked files.
Remove long_u: Passing-by: put_time(): Refactor implementation.

put_time() had a complicated implementation, because of having to shift
an 8-byte value in a portable way with old means.
That can be greatly simplified now, using a C99 fixed-size type.