MEDIUM: sock_raw: introduce a read0 callback that is different from shutr

MEDIUM: sock_raw: introduce a read0 callback that is different from shutr

This one is supposed to be called by the lower layer upon receiving a shutr
notification, which is different from the call performed by the upper layer.
Specifically, this function will ultimately not call EV_FD_* but will just
manipulate event flags instead. The function also does not call shutw anymore
and instead performs the necessary work.

Splitting it into si-specific part and data-specific parts will not be easy.