posix: set task name as argv[0] to match Nuttx
posix: set task name as argv[0] to match Nuttx

This changes px4_task_spawn_cmd to match the NuttX task_spawn. It adds
the task name as argv[0]. See example below:

px4_task_spawn_cmd("task_name",
		   SCHED_DEFAULT,
		   SCHED_PRIORITY_DEFAULT,
		   1024,
		   (px4_main_t)&Something::start_helper,
		   (char *const *)argv);

with:

argv[0]: "something"
argv[1]: "start"
argv[2]: nullptr

becomes in Something::start_helper:

argv[0]: "task_name"
argv[1]: "something"
argv[2]: "start"
argv[3]: nullptr

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