Followup to previous commit; change fd_mask from signed to unsigned as

it should have been to start with.  The implications of that are not
pleasant when combined with >> as it breaks on multiples of NFDBITS.
Right shifting of a signed value fills with a copy of the sign bit,
not a zero.

Reported by:	bde

sys/sys/types.h

@@ -167,7 +167,7 @@ typedef	_BSD_TIMER_T_	timer_t;
 #define	FD_SETSIZE	1024
 #endif
 
-typedef	long	fd_mask;
+typedef	unsigned long	fd_mask;
 #define	NFDBITS	(sizeof(fd_mask) * NBBY)	/* bits per mask */
 
 #ifndef howmany