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>

lib/shadowio.c

@@ -164,7 +164,7 @@ int spw_open (int mode)
 
 int spw_update (const struct spwd *sp)
 {
-	return commonio_update (&shadow_db, (const void *) sp);
+	return commonio_update (&shadow_db, sp);
 }
 
 int spw_remove (const char *name)