core: allow marking a device unmanaged by its interface name

Virtual devices often don't have stable hardware addresses. So allow
for marking a device unmanaged via its interface name instead.

https://bugzilla.gnome.org/show_bug.cgi?id=693684

src/NetworkManagerUtils.h

@@ -41,6 +41,7 @@ void nm_utils_merge_ip6_config (NMIP6Config *ip6_config, NMSettingIP6Config *set
 
 gboolean nm_match_spec_hwaddr (const GSList *specs, const char *hwaddr);
 gboolean nm_match_spec_s390_subchannels (const GSList *specs, const char *subchannels);
+gboolean nm_match_spec_interface_name (const GSList *specs, const char *interface_name);
 
 const char *nm_utils_get_shared_wifi_permission (NMConnection *connection);