The spawn proxy thread is a special existence in NuttX, usually some developers
spend a lot of time on stack overflow of spawn proxy thread:
https://github.com/apache/nuttx/issues/9046https://github.com/apache/nuttx/pull/9081
In order to avoid similar issues, this PR will remove spawn proxy thread to simplify
the process of task/posix_spawn().
1. Postpone the related processing of spawn file actions until after task_init()
2. Delete the temporary thread of spawn proxy and related global variables
Signed-off-by: chao an <anchao@xiaomi.com>