freetdm: core - moved validation of sigstatus from implementation to core, on sigstatus getters/setters
         core - locking the channel on ftdm_channel_get_sig_status() and ftdm_channel_set_sig_status(),
                but not on ftdm_span_get_sig_status() nor ftdm_span_set_sig_status()
         ftmod_r2 - locking channels on ftdm_r2_get_span_sig_status() and ftdm_r2_set_span_sig_status()