Reap jobs before calling select() in job_continue()

Prior to b0e09303a, simple jobs like `printf "%s\n" $line | read word _`
never hit the call to select() because they were reaped in the SIGCHLD
signal handler. With that commit, the signal handler no longer reaps
children, and a job like that would enter select() and hit the 10000μs
timeout before discovering that the job was already complete.

Fixes #1884.

proc.cpp

@@ -1012,6 +1012,9 @@ static int select_try(job_t *j)
         tv.tv_usec=10000;
 
         retval =select(maxfd+1, &fds, 0, 0, &tv);
+        if (retval == 0) {
+            debug(3, L"select_try hit timeout\n");
+        }
         return retval > 0;
     }
 
@@ -1224,6 +1227,9 @@ void job_continue(job_t *j, bool cont)
         {
             bool quit = false;
 
+            /* Look for finished processes first, to avoid select() if it's already done. */
+            process_mark_finished_children(false);
+
             /*
                Wait for job to report.
             */