Syscall API: thread-control-block

• need per-thread pointer that userspace controls and can use in an async-signal-safe way to find errno

• new syscalls: __set_tcb() and __get_tcb()

• pid_t __tfork(struct __tfork { void *tf_stack; void *tf_tcb; ... } *params)

• eventually will also be used for ELF Thread-Local-Storage

• for now, just gives a fast way to find the errno storage for the thread