settings: make connections always have s_ip4 and s_ip6

Make sure that all connections returned from NMSettings or created via
AddAndActivateConnection have an NMSettingIP4Config and an
NMSettingIP6Config, with non-NULL methods, and get rid of
now-unnecessary checks for those.

Also move the slaves-can't-have-IP-config checks into the
platform-independent code as well. This also gets rid of spurious
"ignoring IP4/IP6 configuration" warnings in ifcfg-rh when reading a
slave ifcfg file.

Partly based on a patch from Pavel.

https://bugzilla.gnome.org/show_bug.cgi?id=708875

src/NetworkManagerUtils.h

@@ -82,6 +82,9 @@ gboolean nm_utils_get_proc_sys_net_value_with_bounds (const char *path,
                                                       gint32 valid_min,
                                                       gint32 valid_max);
 
+void nm_utils_normalize_connection (NMConnection *connection,
+                                    gboolean      default_enable_ipv6);
+
 void nm_utils_complete_generic (NMConnection *connection,
                                 const char *ctype,
                                 const GSList *existing,