unix(4): Add SOL_LOCAL:LOCAL_CREDS_PERSISTENT

This option is intended to be semantically identical to Linux's
SOL_SOCKET:SO_PASSCRED.  For now, it is mutually exclusive with the
pre-existing sockopt SOL_LOCAL:LOCAL_CREDS.

Reviewed by:	markj (penultimate version)
Differential Revision:	https://reviews.freebsd.org/D27011

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

@@ -67,6 +67,7 @@ struct sockaddr_un {
 /* Socket options. */
 #define	LOCAL_PEERCRED		1	/* retrieve peer credentials */
 #define	LOCAL_CREDS		2	/* pass credentials to receiver */
+#define	LOCAL_CREDS_PERSISTENT	3	/* pass credentials to receiver */
 #define	LOCAL_CONNWAIT		4	/* connects block until accepted */
 
 /* Start of reserved space for third-party socket options. */