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/copydir.c

@@ -768,7 +768,7 @@ static ssize_t full_write(int fd, const void *buf, size_t count) {
 
 		written += res;
 		buf = (const unsigned char*)buf + res;
-		count -= (size_t)res;
+		count -= res;
 	}
 
 	return written;
@@ -850,7 +850,7 @@ static int copy_file (const struct path_info *src, const struct path_info *dst,
 			break;
 		}
 
-		if (full_write (ofd, buf, (size_t)cnt) < 0) {
+		if (full_write (ofd, buf, cnt) < 0) {
 			(void) close (ofd);
 			(void) close (ifd);
 			return -1;