RFE: export filter in a C header file format

[pcmoore]

Add functionality to export the seccomp BPF filter in a C header file format suitable for including in a project. Ideally the generated C header would include the filter loading function which would handle the prcrtl()/seccomp() call as well as any NNP settings as defined by the filter.

Proposed new libseccomp API:

API int seccomp_export_chdr(const scmp_filter_ctx ctx, int fd);