shell: update shell_task_create() to take a shell argument

Problem: The shell_task_create() function takes a `struct shell_info *`
argument, but the function may need access to the outer job shell
object in the future, and struct shell_info has no pointer to this
object.

Update shell_task_create() to take a flux_shell_t argument.

Author: grondo

src/shell/shell.c

@@ -1668,7 +1668,7 @@ int main (int argc, char *argv[])
     while (taskid != IDSET_INVALID_ID) {
         struct shell_task *task;
 
-        if (!(task = shell_task_create (shell.info, i, taskid)))
+        if (!(task = shell_task_create (&shell, i, taskid)))
             shell_die (1, "shell_task_create index=%d", i);
 
         task->pre_exec_cb = shell_task_exec;

src/shell/task.c

@@ -91,10 +91,11 @@ struct shell_task *shell_task_new (void)
     return NULL;
 }
 
-struct shell_task *shell_task_create (struct shell_info *info,
+struct shell_task *shell_task_create (flux_shell_t *shell,
                                       int index,
                                       int taskid)
 {
+    struct shell_info *info = shell->info;
     struct shell_task *task;
     const char *key;
     json_t *entry;

src/shell/task.h

@@ -54,7 +54,7 @@ struct shell_task {
 
 void shell_task_destroy (struct shell_task *task);
 
-struct shell_task *shell_task_create (struct shell_info *info,
+struct shell_task *shell_task_create (flux_shell_t *shell,
                                       int index,
                                       int taskid);