lwIP crashes after a client connects and sh prints the prompt.

umemcpy is as safe as argptr.

scale-amd64 passes all "argint" syscall arguments via registers, so
it might be more convenient to pass them all to the sys_* functions
(instead of reading them via argint).

This supports width, precision, length, and padding.

lwIP can't run (or be initialized) until the kernel creates the
scheduler thread, so create a worker thread that inits lwIP.