core: add nm_utils_setpgid(), and use it

Add nm_utils_setpgid() as a g_spawn*() child setup function for
calling setpgid(), and use it where appropriate rather than
reimplementing it every time.

src/NetworkManagerUtils.h

@@ -210,4 +210,6 @@ GHashTable *nm_utils_connection_dict_to_hash (GVariant *dict);
 GSList *nm_utils_ip4_routes_from_gvalue (const GValue *value);
 GSList *nm_utils_ip6_routes_from_gvalue (const GValue *value);
 
+void nm_utils_setpgid (gpointer unused);
+
 #endif /* __NETWORKMANAGER_UTILS_H__ */