shell: support per-task output

Problem: The job shell output plugin does not support per-task or
per-node output.

Support opening per-task output via a new task_output abstraction in
output/task.[ch].

Detect if output is per-node in config and use this to determine if
output files need to be opened up on ranks other than 0.

src/shell/output/conf.c

@@ -30,6 +30,12 @@
 
 #include "output/conf.h"
 
+static bool template_is_per_shell (flux_shell_t *shell,
+                                   const char *template)
+{
+    return strstr (template, "{{node") || strstr (template, "{{task");
+}
+
 static int output_stream_getopts (flux_shell_t *shell,
                                   const char *name,
                                   struct output_stream *stream)
@@ -54,8 +60,10 @@ static int output_stream_getopts (flux_shell_t *shell,
         stream->type = FLUX_OUTPUT_TYPE_KVS;
         return 0;
     }
-    if (stream->template)
+    if (stream->template) {
         stream->type = FLUX_OUTPUT_TYPE_FILE;
+        stream->per_shell = template_is_per_shell (shell, stream->template);
+    }
 
     if (strcasecmp (stream->buffer_type, "none") == 0)
         stream->buffer_type = "none";

src/shell/output/conf.h

@@ -26,7 +26,7 @@ struct output_stream {
     const char *template;
     const char *mode;
     bool label;
-    struct file_entry *fp;
+    bool per_shell;
 };
 
 struct output_config {