Handle OpenBSD routes with locked MTU.

In OpenBSD the MTU value of a route may be locked.  This is indicated
by an ' L' in the netstat -r output.  For scapy's read_routes()
this looks like an additional column.  Make the parser aware of
this possibility when finding the network interface.

scapy/arch/unix.py

@@ -78,7 +78,8 @@ def read_routes():
         else:
             rt = l.split()
             dest,gw,flg = rt[:3]
-            netif = rt[4 + mtu_present + prio_present + refs_present]
+            locked = OPENBSD and rt[6] == "L"
+            netif = rt[4 + mtu_present + prio_present + refs_present + locked]
         if flg.find("Lc") >= 0:
             continue
         if dest == "default":