Stop boundary linestrings from relations being used

osm2pgsql will create linestrings for relations in planet_osm_roads (and
potentially planet_osm_line for incomplete/broken relations)

This leads to two problems

* Duplication of the lines when a boundary is complete and does not adjoin
another administrative area

* Overlapping lines

Fixes gravitystorm#344

This makes boundary lines substantially lighter as there are no longer many many
lines stacked on top of each other.

project.mml

@@ -975,7 +975,7 @@
       ],
       "Datasource": {
         "type": "postgis",
-        "table": "(select way,admin_level\n       from planet_osm_roads\n       where \"boundary\"='administrative'\n         and admin_level in ('0','1','2','3','4')\n       ) as admin_01234",
+        "table": "(select way,admin_level\n       from planet_osm_roads\n       where \"boundary\"='administrative'\n         and admin_level in ('0','1','2','3','4')\n         and osm_id > 0\n       ) as admin_01234",
         "extent": "-20037508,-19929239,20037508,19929239",
         "key_field": "",
         "geometry_field": "way",
@@ -998,7 +998,7 @@
       ],
       "Datasource": {
         "type": "postgis",
-        "table": "(select way,admin_level\n       from planet_osm_roads\n       where \"boundary\"='administrative'\n         and admin_level in ('5','6','7','8')\n       ) as admin_5678",
+        "table": "(select way,admin_level\n       from planet_osm_roads\n       where \"boundary\"='administrative'\n         and admin_level in ('5','6','7','8')\n         and osm_id > 0\n       ) as admin_5678",
         "extent": "-20037508,-19929239,20037508,19929239",
         "key_field": "",
         "geometry_field": "way",
@@ -1021,7 +1021,7 @@
       ],
       "Datasource": {
         "type": "postgis",
-        "table": "(select way,admin_level\n       from planet_osm_roads\n       where \"boundary\"='administrative'\n         and (admin_level is null or admin_level not in ('0','1','2','3','4','5','6','7','8'))\n       ) as admin_other",
+        "table": "(select way,admin_level\n       from planet_osm_roads\n       where \"boundary\"='administrative'\n         and (admin_level is null or admin_level not in ('0','1','2','3','4','5','6','7','8'))\n         and osm_id > 0\n       ) as admin_other",
         "extent": "-20037508,-19929239,20037508,19929239",
         "key_field": "",
         "geometry_field": "way",