socket: Implement SO_RERROR

SO_RERROR indicates that receive buffer overflows should be handled as
errors. Historically receive buffer overflows have been ignored and
programs could not tell if they missed messages or messages had been
truncated because of overflows. Since programs historically do not
expect to get receive overflow errors, this behavior is not the
default.

This is really really important for programs that use route(4) to keep
in sync with the system. If we loose a message then we need to reload
the full system state, otherwise the behaviour from that point is
undefined and can lead to chasing bogus bug reports.

Reviewed by:	philip (network), kbowling (transport), gbe (manpages)
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D26652

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

@@ -139,6 +139,7 @@ typedef	__uintptr_t	uintptr_t;
 #define	SO_NO_OFFLOAD	0x00004000	/* socket cannot be offloaded */
 #define	SO_NO_DDP	0x00008000	/* disable direct data placement */
 #define	SO_REUSEPORT_LB	0x00010000	/* reuse with load balancing */
+#define	SO_RERROR	0x00020000	/* keep track of receive errors */
 
 /*
  * Additional options, not kept in so_options.