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/tz.c

@@ -34,7 +34,7 @@
 
 	fp = fopen (fname, "r");
 	if (   (NULL == fp)
-	    || (fgets (tzbuf, (int) sizeof (tzbuf), fp) == NULL)) {
+	    || (fgets (tzbuf, sizeof (tzbuf), fp) == NULL)) {
 		def_tz = getdef_str ("ENV_TZ");
 		if ((NULL == def_tz) || ('/' == def_tz[0])) {
 			def_tz = "TZ=CST6CDT";