* dec_5100.c: sysconf, interrupt, and motherboard (drain writebuffer)
support for 5100.
* support for hardware kludge in 5100: sii DMA buffer is hardware
padded to alternate 32-bit words, not alternating 16-bit halfwords.
* 5100 has no framebuffer, console is wired to serial port 0.
With ibus support, boots as far as exec'ing init, and hangs.