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