Remove dependency on select and poll and switch to non-blocking I/O

For better compatibility with MinGW
Fix possible cross-confusion between serial / usb modes
Code to set time a nonce is received was overlooked
Update Zeus documentation for MinGW
Fix work update requests being ignored.

This was accidentally left in from early development, should
have been removed.
Lower minimum Zeus clock frequency to just act as sanity check
Improvements to Zeus driver read handling
Merge Zeus driver updates from 'zeus_mingw' branch
Add Zeus miners to udev rules