Define box interface.
Do not use unportable break-thread.
Remove break-thread.

break-thread is not needed here, as long running requests will
be interruptted when force-closing ports.
Do not use kill-thread.
Use `th1' as a flag instead of `dead'.
Remove now unneeded thread interfaces.
Implement a simplified version of mutex interface in srfi-18.
Use mutex instead of semaphore to implement atomic-invoke.
`setuid' is not needed when running server by non-root user.