sockets: define shutdown(2) constants in cpp namespace

There is software that uses SHUT_RD, SHUT_WR as preprocessor defines and
its build was broken by enum declaration.  Keep the enum, but provide
defines to propagate the constants to cpp namespace.

PR:		277994
PR:		277995
Fixes:		c3276e02be

sys/sys/socket.h

@@ -629,8 +629,11 @@ struct omsghdr {
  */
 enum shutdown_how {
 	SHUT_RD = 0,		/* shut down the reading side */
+#define	SHUT_RD		SHUT_RD
 	SHUT_WR,		/* shut down the writing side */
+#define	SHUT_WR		SHUT_WR
 	SHUT_RDWR		/* shut down both sides */
+#define	SHUT_RDWR	SHUT_RDWR
 };
 
 #if __BSD_VISIBLE