MEDIUM: connection: Add conn_upgrade_mux_fe() to handle mux upgrades

MEDIUM: connection: Add conn_upgrade_mux_fe() to handle mux upgrades

This function will handle mux upgrades, for frontend connections only. It will
retrieve the best mux in the same way than conn_install_mux_fe except that the
mode and optionnally the proto are forced.

The new multiplexer is initialized using a new context and a specific input
buffer. Then, the old one is destroyed. If an error occurred, everything is
rolled back.