[multistream-select] Re-enable use of ls for the dialer aka "parallel negotiation".

[romanb]

This is just a follow-up ticket for #1795 to not forget about it. The wire format has been fixed and ls responses are now interoperable, but we still need to eventually re-enable the use of ls on the side of the dialer, i.e. make active use of it as before. Its use was temporarily disabled to permit an upgrade path. This should not be done before #1811 has already been released.