MINOR: quic: prepare support for options on FE/BE side

MINOR: quic: prepare support for options on FE/BE side

A major reorganization of QUIC settings is going to be performed. One of
its objective is to clearly define options which can be separately
configured on frontend and backend proxy sides.

To implement this, quic_tune structure is extended to support fe and be
options. A set of macros/functions is also defined : it allows to
retrieve an option defined on both sides with unified code, based on
proxy side of a quic_conn/connection instance.