provide properties attr to services

properties are similar to outputs, but are different in that they are
fixed values (do not change) and are present even when the service is
down

if the attribute is present and an attrset, this will write the
equivalent recursive directory structure to $out/.properties/

pkgs/liminix-tools/services/builder.sh

@@ -1,5 +1,15 @@
 source $stdenv/setup
 mkdir -p $out/${name}
+
+writepath(){
+    mkdir -p $(dirname $1)
+    echo $2 > $1
+}
+if test -n "$propertiesText"; then
+    mkdir $out/.properties
+    ( cd $out/.properties; eval "$propertiesText" )
+fi
+
 echo $serviceType > $out/${name}/type
 mkdir -p $out/${name}/dependencies.d
 echo $buildInputs > $out/buildInputs