Change the logic of open line commands so that if a selection lies
on the end of line character of the line from which we open a new
line, that selection does not move.

If we have two clients, A and B, with B's cursor on the eol character
of line L, and A hits `o` while on line L, B's cursor should stay
on the same (logical) line. Previous behaviour would make B's cursor
jump on the newly inserted line.