Enable libusb when compiling Zeus driver to restore --ndevs
Tweak detect function to be faster
First attempt at Zeus auto-detection, prep for per-device settings
Test calculating a live hashrate
Minor changes in TUI and API output
Silence a gcc compiler warning
Link against libudev for easier Zeus device detection.

For the moment the Zeus driver is not using libusb for
actual I/O. Libudev allows us to associate /dev/ttyN entires
with specific usb bus:device IDs.
Add failsafe check to skip already in-use devices