MINOR: stconn: add sc_is_recv_allowed() to check for ability to receive

MINOR: stconn: add sc_is_recv_allowed() to check for ability to receive

At plenty of places we combine multiple flags checks to determine if we
can receive (endp_ready, rx_blocked, cf_shutr etc). Let's group them
under a single function that is meant to replace existing tests.

Some tests were only checking the rxblk flags at the connection level,
so for now they were not converted, this requires a bit of auditing
first, and probably a test to determine whether or not to check for
cf_shutr (e.g. there is none if no stream is present).