doc: Update build-openbsd for 0.13.0+ and OpenBSD 5.9
- Python 3 now supported.
- Bump boost version to 1.61 - one boost patch no longer needed.
- All checked with OpenBSD 5.9, except for the clang part, I left this as-is for someone adventurous.
- Mention overriding resource limits, OpenBSD's default ulimit does not suffice for building Bitcoin Core with gcc 4.9.3.