MINOR: stconn: Add support for flags during zero-copy forwarding negotiation

MINOR: stconn: Add support for flags during zero-copy forwarding negotiation

During zero-copy forwarding negotiation, a pseudo flag was already used to
notify the consummer if the producer is able to use kernel splicing or not. But
this was not extensible. So, now we use a true bitfield to be able to pass flags
during the negotiation. NEGO_FF_FL_* flags may be used now.

Of course, for now, there is only one flags, the kernel splicing support on
producer side (NEGO_FF_FL_MAY_SPLICE).