Drop explicit unsigned from FD_SETSIZE constant

FD_SETSIZE is often used as an argument to select or compared with an
integer file descriptor.  Rather than force 3rd party software to add
explicit casts, just make it a plain (int) constant as on other
operating systems.

Previous discussion:
http://lists.freebsd.org/pipermail/freebsd-standards/2012-July/002410.html

sys/sys/select.h

@@ -56,7 +56,7 @@ typedef	__sigset_t	sigset_t;
  * be enough for most uses.
  */
 #ifndef	FD_SETSIZE
-#define	FD_SETSIZE	1024U
+#define	FD_SETSIZE	1024
 #endif
 
 #define	_NFDBITS	(sizeof(__fd_mask) * 8)	/* bits per mask */