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>

contrib/adduser.c

@@ -489,7 +489,7 @@ safeget (char *buf, int maxlen)
   while ((c = getc (stdin)) != EOF && (c != '\n') && (++i < maxlen))
     {
       bad = (!isalnum (c) && (c != '_') && (c != ' '));
-      *(buf++) = (char) c;
+      *(buf++) = c;
     }
   *buf = '\0';