iflib(9): Add support for cloning pseudo interfaces

Part 3 of many ...
The VPC framework relies heavily on cloning pseudo interfaces
(vmnics, vpc switch, vcpswitch port, hostif, vxlan if, etc).

This pulls in that piece. Some ancillary changes get pulled
in as a side effect.

Reviewed by:	shurd@
Approved by:	sbruno@
Sponsored by:	Joyent, Inc.
Differential Revision:	https://reviews.freebsd.org/D15347

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

@@ -162,6 +162,9 @@ struct if_data {
 #define	IFF_STATICARP	0x80000		/* (n) static 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 */
+
+
 /*
  * Old names for driver flags so that user space tools can continue to use
  * the old (portable) names.