* Iraqi PM says priority is to spare civilian lives
* Sunni tribesmen urged to evict al Qaeda fighters from city
* Maliki under challenge from resurgent Sunni militants
By Suadad al-Salhy and Alistair Lyon
BAGHDAD, Jan 12 (Reuters) - Iraqi Prime Minister Nuri al-Maliki on Sunday ruled out a military assault on Falluja, saying he wanted to spare the city more carnage and give Sunni...