core: add nm_match_spec_join() function

We have a special implemenation nm_match_spec_split() to split
a string. We also need the reverse operation to be able to convert
a list of specs to string without loss.

src/NetworkManagerUtils.h

@@ -98,6 +98,7 @@ NMMatchSpecMatchType nm_match_spec_hwaddr (const GSList *specs, const char *hwad
 NMMatchSpecMatchType nm_match_spec_s390_subchannels (const GSList *specs, const char *subchannels);
 NMMatchSpecMatchType nm_match_spec_interface_name (const GSList *specs, const char *interface_name);
 GSList *nm_match_spec_split (const char *value);
+char *nm_match_spec_join (GSList *specs);
 
 const char *nm_utils_get_shared_wifi_permission (NMConnection *connection);