Remove superfluous casts

-  Every non-const pointer converts automatically to void *.
-  Every pointer converts automatically to void *.
-  void * converts to any other pointer.
-  const void * converts to any other const pointer.
-  Integer variables convert to each other.

I changed the declaration of a few variables in order to allow removing
a cast.

However, I didn't attempt to edit casts inside comparisons, since they
are very delicate.  I also kept casts in variadic functions, since they
are necessary, and in allocation functions, because I have other plans
for them.

I also changed a few casts to int that are better as ptrdiff_t.

This change has triggered some warnings about const correctness issues,
which have also been fixed in this patch (see for example src/login.c).

Signed-off-by: Alejandro Colomar <alx@kernel.org>

libsubid/api.c

@@ -66,12 +66,12 @@ int get_subid_owner(unsigned long id, enum subid_type id_type, uid_t **owner)
 
 int subid_get_uid_owners(uid_t uid, uid_t **owner)
 {
-	return get_subid_owner((unsigned long)uid, ID_TYPE_UID, owner);
+	return get_subid_owner(uid, ID_TYPE_UID, owner);
 }
 
 int subid_get_gid_owners(gid_t gid, uid_t **owner)
 {
-	return get_subid_owner((unsigned long)gid, ID_TYPE_GID, owner);
+	return get_subid_owner(gid, ID_TYPE_GID, owner);
 }
 
 static