Merge branch 'Make BPF skeleton easier to use from C++ code'

Andrii Nakryiko says:

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Add minimal C++-specific additions to BPF skeleton codegen to facilitate
easier use of C skeletons in C++ applications. These additions don't add any
extra ongoing maintenance and allows C++ users to fit pure C skeleton better
into their C++ code base. All that without the need to design, implement and
support a separate C++ BPF skeleton implementation.

v1->v2:
  - use default argument values in T::open() (Alexei).
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Signed-off-by: Alexei Starovoitov <ast@kernel.org>