bpf: make tc's bpf loader generic and move into lib

bpf: make tc's bpf loader generic and move into lib

This work moves the bpf loader into the iproute2 library and reworks
the tc specific parts into generic code. It's useful as we can then
more easily support new program types by just having the same ELF
loader backend. Joint work with Thomas Graf. I hacked a rough start
of a test suite to make sure nothing breaks [1] and looks all good.

  [1] https://github.com/borkmann/clsact/blob/master/test_bpf.sh

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Thomas Graf <tgraf@suug.ch>