hameg-hmo: Move the declaration of the driver info out of protocol.h

This fixes duplicate symbol error on Mac OS X.

BugLink: http://sigrok.org/bugzilla/show_bug.cgi?id=216
serial-dmm: Commented struct dmm_info.
serial-dmm: Add BBC Goerz Metrawatt M2110 DMM driver.
bbcgm-m2110: Minor cosmetics.
Bump minimum libusb version to 1.0.16

This gets us the libusb version checking mechanism itself, hopefully
making this sort of thing easier in future.

Also hotplug, device tree traversal, and lots of fixes.