MINOR: connection: add a new CO_FL_WANT_DRAIN flag to force drain on close

MINOR: connection: add a new CO_FL_WANT_DRAIN flag to force drain on close

Sometimes we'd like to do our best to drain pending data before closing
in order to save the peer from risking to receive an RST on close.

This adds a new connection flag CO_FL_WANT_DRAIN that is used to
trigger a call to conn_ctrl_drain() from conn_ctrl_close(), and the
sock_drain() function ignores fd_recv_ready() if this flag is set,
in order to catch latest data. It's not used for now.