Micko: sm7238 is missing a few features and has video errors but otherwise ready to submit.

pdp11 stuff is harder; 1801VM2 core has to be cleanly split from T11 core, and needs a better way to handle bus errors than making execute_set_input public. I'll push my wip tree so you can see why smile

In my wip I've added basic flow control to null_modem and I think this exposed a bug in i8251 (writes at https://github.com/mamedev/mame/blob/master/src/devices/machine/i8251.cpp#L419 shouldn't be inverted), maybe others have it wrong too.