Add support for libbpf API compatibility about ebpf-for-windows

[dthaler]

This is to expose literally the libbpf APIs, at least as .h file inline wrappers if not native.

[dthaler]

libbpf APIs have a permissive license so some of the code may be usable, even if not the code that wraps bpf() syscall.

[dthaler]

There are some issues currently in the libbpf code base, including:

1. GCC compiler-specific pragmas and __attribute__ markups
2. Direct inclusion of <linux/…> headers
3. GCC-compiler specific language construct “= {}” instead of “= {0}”
4. GCC-compiler pecific language construct "?:", as in "err = err ?: value;"
5. Deprecated APIs like strdup instead of _strdup
6. Files with ambiguous license (no header at top) like https://raw.githubusercontent.com/libbpf/libbpf/master/travis-ci/vmtest/vmlinux.h

The above could only be addressed by updating the libbpf code itself.

[dthaler]

After much investigation, I've concluded that it would not currently make sense to try to use libbpf code directly, since libbpf hard-codes a list of prog types, helper func id names, etc., which lists can vary by platform. So the current libbpf implementation is really Linux-specific even if the header file is platform agnostic. At least in the short term, runtime implementations would be better off implementing their own platform-specific implementations of the common libbpf header.