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/getgr_nam_gid.c

@@ -36,7 +36,7 @@ extern /*@only@*//*@null@*/struct group *getgr_nam_gid (/*@null@*/const char *gr
 	    && ('\0' == *endptr)
 	    && (ERANGE != errno)
 	    && (/*@+longintegral@*/gid == (gid_t)gid)/*@=longintegral@*/) {
-		return xgetgrgid ((gid_t) gid);
+		return xgetgrgid (gid);
 	}
 	return xgetgrnam (grname);
 }