make NET_TASKQ optionable

[Chris Cappuccio]

For future testing purposes, is this ok for the tree?

Index: if.c
===================================================================
RCS file: /cvs/src/sys/net/if.c,v
retrieving revision 1.570
diff -u -p -u -r1.570 if.c
--- if.c        20 Dec 2018 10:26:36 -0000      1.570
+++ if.c        30 Dec 2018 18:49:41 -0000
@@ -233,7 +233,9 @@ int ifq_congestion;
 
 int             netisr;
 
+#ifndef NET_TASKQ
 #define        NET_TASKQ       1
+#endif
 struct taskq   *nettqmp[NET_TASKQ];
 
 struct task if_input_task_locked = TASK_INITIALIZER(if_netisr, NULL);