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/addgrps.c

@@ -100,7 +100,7 @@ int add_groups (const char *list)
 	}
 
 	if (added) {
-		ret = setgroups ((size_t)ngroups, grouplist);
+		ret = setgroups (ngroups, grouplist);
 		free (grouplist);
 		return ret;
 	}