ip: allow using a device "help" (or a prefix thereof)

Device names that match "help" or a prefix thereof should be allowed anywhere
a device name can be used. Note that a suitable keyword ("dev" or "name", the
latter for "ip tunnel") has to be used in these cases to resolve ambiguities.
Signed-off-by: Christoph Schulz <develop@kristov.de>
Reported-by: Leonhard Preis <leonhard@pre.is>
Reported-by: Wilhelm Wijkander <lists@0x5e.se>

ip/ip6tunnel.c

@@ -287,7 +287,7 @@ static int parse_args(int argc, char **argv, int cmd, struct ip6_tnl_parm2 *p)
 			if (strcmp(*argv, "name") == 0) {
 				NEXT_ARG();
 			}
-			if (matches(*argv, "help") == 0)
+			else if (matches(*argv, "help") == 0)
 				usage();
 			if (p->name[0])
 				duparg2("name", *argv);

ip/ipaddress.c

@@ -1580,7 +1580,7 @@ static int ipaddr_list_flush_or_save(int argc, char **argv, int action)
 			if (strcmp(*argv, "dev") == 0) {
 				NEXT_ARG();
 			}
-			if (matches(*argv, "help") == 0)
+			else if (matches(*argv, "help") == 0)
 				usage();
 			if (filter_dev)
 				duparg2("dev", *argv);

ip/iplink.c

@@ -1148,8 +1148,7 @@ static int do_set(int argc, char **argv)
 		} else {
 			if (strcmp(*argv, "dev") == 0)
 				NEXT_ARG();
-
-			if (matches(*argv, "help") == 0)
+			else if (matches(*argv, "help") == 0)
 				usage();
 
 			if (dev)

ip/ipmaddr.c

@@ -257,7 +257,7 @@ static int multiaddr_list(int argc, char **argv)
 			if (strcmp(*argv, "dev") == 0) {
 				NEXT_ARG();
 			}
-			if (matches(*argv, "help") == 0)
+			else if (matches(*argv, "help") == 0)
 				usage();
 			if (filter.dev)
 				duparg2("dev", *argv);