Prevent a second PMI2_Init call from leaving a hung slurmstepd process.

A second PMI2_Init() within the same step is invalid, and cannot succeed.

Return an error code back to the client end, and close the fd to force the
step to terminate immediately.

Due to a bug in our libpmi code, just returning a cmd=response_to_init with
an appropriate rc number will not tear down the connection properly, so
send back something else that will trigger the error path.

Bug 3520.