By Saliou Samb CONAKRY (Reuters) - One person was killed and about were 10 injured when security forces and supporters of Guinea's president clashed with protesters marching in the capital on Thursday against planned legislative elections. President Alpha Conde's opponents say he plans to rig the long-delayed polls due to take place on June 30. Conde took office in 2010 following the first democratic transfer of power in the mineral-rich nation since independence in 1958. ...