lwIP/net/e1000 seem to have a few nagging bugs, but httpd is able to
serve files.

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.