MINOR: stream-int: introduce new SI_FL_RXBLK flags

MINOR: stream-int: introduce new SI_FL_RXBLK flags

The plan is to have the following flags to describe why a stream interface
doesn't produce data :

    - SI_FL_RXBLK_CHAN : the channel doesn't want it to receive
    - SI_FL_RXBLK_BUFF : waiting for a buffer allocation to complete
    - SI_FL_RXBLK_ROOM : more room is required in the channel to receive
    - SI_FL_RXBLK_SHUT : input now closed, nothing new will come
    - SI_FL_RX_WAIT_EP : waiting for the endpoint to produce more data

Applets like the CLI which consume complete commands at once and produce
large chunks of responses will for example be able to stop being woken up
by clearing SI_FL_WANT_GET and setting SI_FL_RXBLK_ROOM when the rx buffer
is full. Once called they will unblock WANT_GET. The flags were moved
together in readable form with the Rx bits using 2 hex digits and still
have some room to do a similar operation on the Tx path later, with the
WAIT_EP flag being represented alone on a digit.