Implement file-based version numbers

This moves the various bpf.o files to have a version-ish prefix. The idea here is that for any given foo.bpf.o we can have multiple versions for different kernels, with udev-hid-bpf loading those in descending order until one of them succeeds. For example we will attempt to load 30-foo.bpf.o first, if that fails we'll try 20-foo.bpf.o and if that succeeds ignore 10-foo.bpf.o (assuming that all three are assigned to a device).

A semi-automated testcase is available in test/uhid-test.sh which can test the path loading and udev loading in a somewhat repeatable manner, run with test/uhid-test.sh test-udev-load for example.

Closes #15 (closed), #18 (closed)