* Obama says law would complicate relations with Uganda
* U.S. sends more than $400 million a year to Uganda
* Homosexuality a taboo in many African countries
By Elias Biryabarema
KAMPALA, Feb 18 (Reuters) - Uganda on Tuesday dismissed U.S. president Barack Obama's call to its leader Yoweri Museveni not to sign an anti-homosexuality law, saying the U.S. was trying to blackmail the east ...