iplink: support show and set of "addrgenmode random"

"random" is a new IPv6 addrgenmode, enabling "stable_secret" type
addresses with an auto-generated secret.

$ ip link set eth0 addrgenmode random

$ ip -d link show dev eth0
2: eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast state DOWN mode DEFAULT group default qlen 1000
    link/ether 00:21:86:a3:25:7d brd ff:ff:ff:ff:ff:ff promiscuity 0 addrgenmode random

Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Bjørn Mork <bjorn@mork.no>

ip/ipaddress.c

@@ -296,6 +296,9 @@ static void print_af_spec(FILE *fp, struct rtattr *af_spec_attr)
 		case IN6_ADDR_GEN_MODE_STABLE_PRIVACY:
 			fprintf(fp, "addrgenmode stable_secret ");
 			break;
+		case IN6_ADDR_GEN_MODE_RANDOM:
+			fprintf(fp, "addrgenmode random ");
+			break;
 		default:
 			fprintf(fp, "addrgenmode %#.2hhx ", mode);
 			break;

ip/iplink.c

@@ -84,7 +84,7 @@ void iplink_usage(void)
 	fprintf(stderr, "				   [ state { auto | enable | disable} ] ]\n");
 	fprintf(stderr, "			  [ master DEVICE ]\n");
 	fprintf(stderr, "			  [ nomaster ]\n");
-	fprintf(stderr, "			  [ addrgenmode { eui64 | none | stable_secret } ]\n");
+	fprintf(stderr, "			  [ addrgenmode { eui64 | none | stable_secret | random } ]\n");
 	fprintf(stderr, "	                  [ protodown { on | off } ]\n");
 	fprintf(stderr, "       ip link show [ DEVICE | group GROUP ] [up] [master DEV] [type TYPE]\n");
 
@@ -178,6 +178,8 @@ static int get_addr_gen_mode(const char *mode)
 		return IN6_ADDR_GEN_MODE_NONE;
 	if (strcasecmp(mode, "stable_secret") == 0)
 		return IN6_ADDR_GEN_MODE_STABLE_PRIVACY;
+	if (strcasecmp(mode, "random") == 0)
+		return IN6_ADDR_GEN_MODE_RANDOM;
 	return -1;
 }