rsocket: Handle SHUT_RD/WR shutdown flags

rsocket: Handle SHUT_RD/WR shutdown flags

Sridhar Samudrala <sri@us.ibm.com> reported an error (EOPNOTSUPP)
after calling select().

The issue is that rshutdown(SHUT_WR) was called before select().
As part of shutdown, rsockets switches the underlying fd from
nonblocking to blocking to ensure that previously sent data has
completed.  shutdown(SHUT_WR) indicates that the socket should be
kept open for receiving data.

Delay handling the actual shutdown unless SHUT_RDWR is specified,
or the socket is closed.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>