sha256 when openssl < 0.9.7. Patch from djm@. Corrections/testing by me.
SHA384, which we don't need and doesn't compile without tweaks
[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