MINOR: connection: add a new flag CO_FL_FDLESS on fd-less connections

MINOR: connection: add a new flag CO_FL_FDLESS on fd-less connections

QUIC connections do not use a file descriptor, instead they use the
quic equivalent which is the quic_conn. A number of our historical
functions at the connection level continue to unconditionally touch
the file descriptor and this may have consequences once QUIC starts
to be used.

This patch adds a new flag on QUIC connections, CO_FL_FDLESS, to
mention that the connection doesn't have a file descriptor, hence the
FD-based API must never be used on them.

From now on it will be possible to intrument existing functions to
panic when this flag is present.