libsubprocess: allow server write before start

Problem: data written to the subprocess server before the subprocess
has entered RUNNING state is silently discarded, but must be handled
in order to eliminate extraneous buffering on the client size.

Drop stdin data only when the subprocess is in FAILED or EXITED state.

Author: garlick

src/common/libsubprocess/server.c

@@ -440,7 +440,8 @@ static void server_write_cb (flux_t *h,
      * that the log messages end up being a nuisance.
      */
     if (!(p = proc_find_byclient (s, msg))
-        || p->state != FLUX_SUBPROCESS_RUNNING)
+        || p->state == FLUX_SUBPROCESS_FAILED
+        || p->state == FLUX_SUBPROCESS_EXITED)
         goto out;
 
     if (data && len) {