Load balance sockets with new SO_REUSEPORT_LB option

This patch adds a new socket option, SO_REUSEPORT_LB, which allow multiple
programs or threads to bind to the same port and incoming connections will be
load balanced using a hash function.

Most of the code was copied from a similar patch for DragonflyBSD.

However, in DragonflyBSD, load balancing is a global on/off setting and can not
be set per socket. This patch allows for simultaneous use of both the current
SO_REUSEPORT and the new SO_REUSEPORT_LB options on the same system.

Required changes to structures
Globally change so_options from 16 to 32 bit value to allow for more options.
Add hashtable in pcbinfo to hold all SO_REUSEPORT_LB sockets.

Limitations
As DragonflyBSD, a load balance group is limited to 256 pcbs
(256 programs or threads sharing the same socket).

Submitted by:	Johannes Lundberg <johanlun0@gmail.com>
Sponsored by:	Limelight Networks
Differential Revision:	https://reviews.freebsd.org/D11003

newlib/libc/sys/rtems/include/sys/socket.h

@@ -147,6 +147,9 @@ typedef	__uintptr_t	uintptr_t;
 #define	SO_NO_OFFLOAD	0x4000		/* socket cannot be offloaded */
 #define	SO_NO_DDP	0x8000		/* disable direct data placement */
 
+// XXX: so_options was only 16 bit, now globally increased to 32 bit
+#define	SO_REUSEPORT_LB	0x00010000 /* reuse with load balancing */
+
 /*
  * Additional options, not kept in so_options.
  */