Add SOL_LOCAL symbolic constant for unix socket option level.

The constant seems to exists on MacOS X >= 10.8.

Requested by:	swills
Reviewed by:	allanjude, kevans
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D25933

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

@@ -62,6 +62,8 @@ struct sockaddr_un {
 
 #if __BSD_VISIBLE
 
+#define	SOL_LOCAL		0	/* Options for local socket */
+
 /* Socket options. */
 #define	LOCAL_PEERCRED		1	/* retrieve peer credentials */
 #define	LOCAL_CREDS		2	/* pass credentials to receiver */