Introduce tracepoint support in libbpf via new `bpf_attach_tracepoint`
API, which takes the tracepoint category and name (e.g. "sched",
"sched_switch"). Attach the tracing program to the tracepoint's id
and proceed as usual.

Add `attach_tracepoint` API to Python BPF module, which takes the
tracepoint description as a single string (e.g. "sched:sched_switch").
Load the BPF program with bpf_prog_type set to TRACEPOINT and then
call `bpf_attach_tracepoint` to attach it.