src/usermod.c: Use id_t for parsing uid_t and gid_t

src/usermod.c: Use id_t for parsing uid_t and gid_t

Use a static_assert(3) to make sure that id_t == uid_t == gid_t.

And use uintmax_t to print it, since on Linux they are unsigned types.

Link: <https://github.com/shadow-maint/shadow/pull/951>
Reviewed-by: Tycho Andersen <tycho@tycho.pizza>
Cc: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>