* FTSEurofirst 300 flat, Euro STOXX 50 down 0.07 pct * FTEU 300 set to snap longest run of weekly gains in 2 yrs * Euro STOXX 50 finds 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 By Blaise Robinson PARIS, June 27 (Reuters) - European stocks were steady aroun