conf-parser: restrict .include usage

Disallow recursive .include, and make it unavailable in anything but
unit files.

src/core/load-dropin.c

@@ -200,7 +200,7 @@ int unit_load_dropin(Unit *u) {
         STRV_FOREACH(f, u->dropin_paths) {
                 r = config_parse(u->id, *f, NULL,
                                  UNIT_VTABLE(u)->sections, config_item_perf_lookup,
-                                 (void*) load_fragment_gperf_lookup, false, u);
+                                 (void*) load_fragment_gperf_lookup, false, false, u);
                 if (r < 0)
                         return r;
         }

src/core/load-fragment.c

@@ -2270,7 +2270,7 @@ static int load_from_path(Unit *u, const char *path) {
                 /* Now, parse the file contents */
                 r = config_parse(u->id, filename, f, UNIT_VTABLE(u)->sections,
                                  config_item_perf_lookup,
-                                 (void*) load_fragment_gperf_lookup, false, u);
+                                 (void*) load_fragment_gperf_lookup, false, true, u);
                 if (r < 0)
                         goto finish;
 

src/core/main.c

@@ -677,7 +677,7 @@ static int parse_config_file(void) {
                 return 0;
         }
 
-        r = config_parse(NULL, fn, f, "Manager\0", config_item_table_lookup, (void*) items, false, NULL);
+        r = config_parse(NULL, fn, f, "Manager\0", config_item_table_lookup, (void*) items, false, false, NULL);
         if (r < 0)
                 log_warning("Failed to parse configuration file: %s", strerror(-r));