MINOR: raw_sock: make use of CO_FL_WILL_UPDATE

MINOR: raw_sock: make use of CO_FL_WILL_UPDATE

Now when raw_sock_{to,from}_{pipe,buf} are called, if the connection
doesn't feature CO_FL_WILL_UPDATE, they will first retrieve the updated
flags using conn_refresh_polling_flags() before changing any flag, then
call conn_cond_update_sock_polling() before leaving, to commit such
changes. Note that the only real call to one of the __conn_* functions
is in fact in conn_sock_read0() which is called from here.