MEDIUM: listeners: make use of fd_want_recv_safe() to enable early receivers

MEDIUM: listeners: make use of fd_want_recv_safe() to enable early receivers

We used to refrain from calling fd_want_recv() if fd_updt was not allocated
but it's not the right solution as this does not allow the FD to be set.
Instead, let's use the new fd_want_recv_safe() which will update the FD and
create an update entry only if possible. In addition, the equivalent test
before calling fd_stop_recv() was removed as totally useless since there's
not fd_updt creation in this case.