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

Layout build machinery closer to LibreSSL to make code sharing easier.
Split the big Makefile.am into src/compat.

These do not exist on all systems in sys/cdefs.h, and they are unneeded
since the header is being used privately anyway.

This reworks a few of the common patches on Debian and Gentoo to be a
little nicer, and changes the patches to be per-fix rather than
per-file.

include naddy's patch for using the LOG_NTP syslog facility

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.