libusb: change callback register handler to int

[aokblast]

Summary

libusb upstream uses int for register handler. This causes some library
user (like pyusb) to assume that we have int in all implementations and
therefore provides a 4 byte storage only. This causes Segmentation
fault as we will right the pointer.

[aokblast]

Minor fixes

[emaste]

This should be fine to recommit now that the so version has been bumped

[aokblast]

Oh, I can re-commit this version. Let me bring it back later.

[aokblast]

It needs rebase. Do it tomorrow.