pull out enums from main BPF class to avoid namespace collisions

pull out enums from main BPF class to avoid namespace collisions

In #3479, the `bpf_attach_type` enum was pulled into the `BPF` class so
that its members could be used in `attach_func` and `detach_func`
functions introduced to the Python API.

Unfortunately this caused a redefinition of BPF.XDP, which was similarly
pulled in from `bpf_prog_type` enum, thus breaking program loading
(#3489).

Let's pull these enum- and flag-type class variables out into their own
wrapper classes. For backwards compatibility, keep them all (except for
`bpf_attach_type`, which was merged 2 days ago) defined in the BPF
class, but add a comment to not continue doing this.