* FTSEurofirst 300 flat, Euro STOXX 50 down 0.2 pct * Euro STOXX 50 hits support on 50-day moving average * Buying convertibles seen good way to avoid pull-backs * Barclays halts sell-off sparked by U.S. dark pool lawsuit * Reuters poll shows further equity gains expected in H2 By Blaise Robinson PARIS, June 27 (Reuters) - European stocks were steady on Friday,