postal boundary may be imported without name

Postal boundaries usually just have the postcode tag set and are
therefore officially 'nameless'. We want to have them as
boundary=postal_code anyways in order to distiguish them from postcode
points inherited from addr: tags.

settings/import-address.style

@@ -44,7 +44,8 @@
 {
     "keys" : ["boundary"],
     "values" : {
-        "administrative" : "main"
+        "administrative" : "main",
+        "postal_code" : "main"
     }
 },
 {

settings/import-extratags.style

@@ -125,6 +125,7 @@
     "keys" : ["boundary"],
     "values" : {
         "place" : "skip",
+        "postal_code" : "main",
         "" : "main,with_name"
     }
 },

settings/import-full.style

@@ -125,6 +125,7 @@
     "keys" : ["boundary"],
     "values" : {
         "place" : "skip",
+        "postal_code" : "main",
         "" : "main,with_name"
     }
 },