Version 5.0.0-beta8

If we're lucky this will be the last of the betas. The remaining potentially blocking issues have to do with improving SerialPort.list which would change their output. The two issues are #1220 and #1084. I need help on those two issues, if I'm not able to close them soon, I'll release anyway, and they'll be fixed for 6x. This release is large enough. -@reconbot

• [linux] Add the ttyAP subsystem to serialport list thanks to @fly19890211 for reporting it
• [all] Have drain wait for pending JS write operations before calling system drain thanks to @digitalhack for reporting it
• [unix] Move setting up the baudrate to the end of the open() to better support custom baudrates