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Add two simple example BPF schedulers - simple and qmap.

* simple: In terms of scheduling, it behaves identical to not having any
  operation implemented at all. The two operations it implements are only to
  improve visibility and exit handling. On certain homogeneous
  configurations, this actually can perform pretty well.

* qmap: A fixed five level priority scheduler to demonstrate queueing PIDs
  on BPF maps for scheduling. While not very practical, this is useful as a
  simple example and will be used to demonstrate different features.

v7: - Compat helpers stripped out in prepartion of upstreaming as the
      upstreamed patchset will be the baselinfe. Utility macros that can be
      used to implement compat features are kept.

    - Explicitly disable map autoattach on struct_ops to avoid trying to
      attach twice while maintaining compatbility with older libbpf.

v6: - Common header files reorganized and cleaned up. Compat hel...