* U.N. envoy appeals to leaders for calm
* President declares curfew in Juba
* South Sudan has struggled since 2011 independence
By Carl Odera
JUBA, Dec 16 (Reuters) - South Sudan's President Salva Kiir declared a curfew in the capital Juba on Monday after clashes overnight between rival factions of soldiers following months of tension between Kiir and his sacked deputy....