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/port.c

@@ -134,7 +134,7 @@ static struct port *getportent (void)
 	 * Lines which begin with '#' are all ignored.
 	 */
 
-	if (fgets (buf, (int) sizeof buf, ports) == 0) {
+	if (fgets (buf, sizeof buf, ports) == 0) {
 		errno = saveerr;
 		return 0;
 	}