ensure stepd -> srun client socket fully shutdown

ensure eio objects get explicitly shutdown when
eio_handle_mainloop exits. currently depending on
whether the order the eio_handle_mainloop and
eio_signal_shutdown get called relative to each other

when stepd is instructed to shut down the socket use
SHUT_RDWR instead of SHUT_RD. just using SHUT_RD can
cause srun to receive ECONNRESET if there's outstanding
data that's been sent to stepd that the task has not
read.

bug 3166