arp: Implement sticky ARP mode for interfaces.

Provide sticky ARP flag for network interface which marks it as the
"sticky" one similarly to what we have for bridges. Once interface is
marked sticky, any address resolved using the ARP will be saved as a
static one in the ARP table. Such functionality may be used to prevent
ARP spoofing or to decrease latencies in Ethernet networks.

The drawbacks include potential limitations in usage of ARP-based
load-balancers and high-availability solutions such as carp(4).

The implemented option is disabled by default, therefore should not
impact the default behaviour of the networking stack.

Sponsored by:		Conclusive Engineering sp. z o.o.
Reviewed By:		melifaro, pauamma_gundo.com
Differential Revision: https://reviews.freebsd.org/D35314
MFC after:		2 weeks

newlib/libc/sys/rtems/include/net/if.h

@@ -160,6 +160,7 @@ struct if_data {
 #define	IFF_PPROMISC	0x20000		/* (n) user-requested promisc mode */
 #define	IFF_MONITOR	0x40000		/* (n) user-requested monitor mode */
 #define	IFF_STATICARP	0x80000		/* (n) static ARP */
+#define	IFF_STICKYARP	0x100000	/* (n) sticky ARP */
 #define	IFF_DYING	0x200000	/* (n) interface is winding down */
 #define	IFF_RENAMING	0x400000	/* (n) interface is being renamed */
 #define	IFF_NOGROUP	0x800000	/* (n) interface is not part of any groups */