e0ed8ed664 went too far and broke the
backward compat headers for Linux systems.

There are crypto hash function symbol overlaps between libmd and libcrypto on
FreeBSD, which causes hashing of the certificates store to fail as a
side-effect when using TLS constraints. Since all we really need is MD5, just
use the embedded OpenBSD versions instead.

- add closefrom fallback for OS X / Linux systems, extracted from sudo,
but without the optimized versions, since they cannot work in a
chroot environment (and we're not performance critical here.)
- enable detecting libtls
- conditionally enable https constraint support

Fixes #3, thanks to @gitisihara for providing the initial patch and
testing.

Remove dependency on libcrypto, include hash headers in a nicer way.

First attempt at building a new OpenNTPD portable project, based on the
latest OpenBSD source.