MEDIUM: fd: mark the FD as ready when it's inserted

MEDIUM: fd: mark the FD as ready when it's inserted

Given that all our I/Os are now directed from top to bottom and not the
opposite way around, and the FD cache was removed, it doesn't make sense
anymore to create FDs that are marked not ready since this would prevent
the first accesses unless the caller explicitly does an fd_may_recv()
which is not expected to be its job (which conn_ctrl_init() has to do
by the way). Let's move this into fd_insert() instead, and have a single
atomic operation for both directions via fd_may_both().