core: move bpf devices implementation to bpf-devices.[ch] and rename

core: move bpf devices implementation to bpf-devices.[ch] and rename

The naming of the functions was a complete mess: the most specific functions
which don't know anything about cgroups had "cgroup_" prefix, while more
general functions which took a node path and a cgroup for reporting had no
prefix. Let's use "bpf_devices_" for the latter group, and "bpf_prog_*" for the
rest.

The main goal of this move is to split the implementation from the calling code
and add unit tests in a later patch.