Rather than attempt to apply 14 years' worth of changes to OpenBSD's sha2
I imported the current versions directly then re-applied the portability
changes. This also allowed re-syncing digest-libc.c against upstream.
Disables and removes dependency on OpenSSL. Many features don't
work and the set of crypto options is greatly restricted. This
will only work on system with native arc4random or /dev/urandom.
Considered highly experimental for now.
[openbsd-compat/sha2.h openbsd-compat/openbsd-compat.h]
[openbsd-compat/sha2.c] First stab at portability glue for SHA256
KEX support, should work with libc SHA256 support or OpenSSL
EVP_sha256 if present