pid_t rfork_thread(int forkflags, void *stack, int (*func)(void *arg),
void *arg);
A new process is created, presumably using RFMEM shared address space.
The child process switches to the supplied stack, which is set up with a
function call frame. The function is called with the supplied arguement.
If the function returns, the return value will be used with _exit(2).
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