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.