add "waitup", s6 readiness helper for network interfaces

run e.g. "waitup wlan0 10" to wait until wlan0 is operationally
up and running, and then send a newline to file descriptor 10

pkgs/waitup/waitup.fnl

@@ -0,0 +1,36 @@
+(local netlink (require :netlink))
+(local sock (netlink.socket))
+
+(when  (< (# arg) 2)
+  (print "usage: waitup ifname fd")
+  (os.exit 1))
+
+(local ifname (. arg 1))
+(local fd (tonumber (. arg 2)))
+(local stream (io.open (.. "/proc/self/fd/" fd) "w"))
+
+(fn notify-ready []
+  (stream:write "\n")
+  (print (.. (. arg 0) ": received netlink operstate up for " ifname))
+  (stream:close))
+
+(fn run-events [evs]
+  (each [_ v (ipairs evs)]
+    (print :event v.event v.name)
+
+    (match v
+      ;; - up: Reflects the administrative state of the interface (IFF_UP)
+      ;; - running: Reflects the operational state (IFF_RUNNING).
+      {:event "newlink" :name ifname :up :yes :running :yes}
+      (notify-ready)
+
+      {:event "newlink" :name ifname :up :no}
+      (os.exit 0))))
+
+(run-events (sock:query {:link true}))
+
+(print (.. (. arg 0) ": waiting for netlink NEWLINK " ifname))
+
+(while (sock:poll)
+  (let [ev (sock:event)]
+    (run-events ev)))