rfork(2) wrapper for simple rfork-style threads. I have lost count of
the number of times I have given this to people and got asked: why isn't
it in libc? It is impossible to do this without assembler glue to reset
the stack for the new child process.

int rfork_thread(flags, stack_addr, start_fnc, start_arg)
int flags; Flags to rfork system call. See rfork(2).
void *stack_addr; Top of stack for thread.
int (*start_fnc)(void *); Address of thread function to call in child.
void *start_arg; Argument to pass to the thread function in child.

This is deliberately not documented or prototyped in includes until the
corresponding alpha version is written.