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>

libmisc/utmp.c

@@ -115,7 +115,7 @@ static void updwtmp (const char *filename, const struct utmp *ut)
 
 	fd = open (filename, O_APPEND | O_WRONLY, 0);
 	if (fd >= 0) {
-		write (fd, (const char *) ut, sizeof (*ut));
+		write (fd, ut, sizeof (*ut));
 		close (fd);
 	}
 }