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/log.c

@@ -82,7 +82,7 @@ void dolastlog (
 	strncpy (newlog.ll_host, host, sizeof (newlog.ll_host) - 1);
 #endif
 	if (   (lseek (fd, offset, SEEK_SET) != offset)
-	    || (write (fd, (const void *) &newlog, sizeof newlog) != (ssize_t) sizeof newlog)
+	    || (write (fd, &newlog, sizeof newlog) != (ssize_t) sizeof newlog)
 	    || (close (fd) != 0)) {
 		SYSLOG ((LOG_WARN,
 		         "Can't write lastlog entry for UID %lu in %s.",