odhcp-script: delete stale dirs from previous runs

This requires adding LFS as a dependency because native Lua has
no way to iterate a directory, but it seems to be Not Huge and
hopefully we'll have other uses for it

pkgs/odhcp-script/odhcp6-script.fnl

@@ -1,8 +1,25 @@
 (local { : split : merge : mkdir } (require :anoia))
 (local { : view } (require :fennel))
+(local lfs (require :lfs))
+
+(fn rmtree [pathname]
+  (case (lfs.symlinkattributes pathname)
+    nil true
+    {:mode "directory"}
+    (do
+      (each [f (lfs.dir pathname)]
+        (when (not (or (= f ".") (= f "..")))
+          (rmtree ( .. pathname "/" f)))
+        (lfs.rmdir pathname)))
+    {:mode "file"}
+    (os.remove pathname)
+    {:mode "link"}
+    (os.remove pathname)
+    unknown
+    (error (.. "can't remove " pathname " of kind \"" unknown.mode "\""))))
+
 
 (local state-directory (assert (os.getenv "SERVICE_STATE")))
-
 (mkdir state-directory)
 
 (fn write-value [name value]
@@ -13,7 +30,6 @@
 (fn write-value-from-env [name]
   (write-value name (os.getenv (string.upper name))))
 
-
 (fn parse-address [str]
   (fn parse-extra [s]
     (let [out {}]
@@ -39,6 +55,10 @@
 ;; a half-updated snapshot
 (os.remove (.. state-directory "/state"))
 
+;; remove parsed addresses/prefixes from any previous run
+(rmtree (.. state-directory "/prefix"))
+(rmtree (.. state-directory "/address"))
+
 (let [wanted
       [
        :addresses