[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
openbsd-compat/Makefile.in] support compression on platforms that
have no/broken MAP_ANON. Moved code to openbsd-compat/xmmap.c
Based on patch from nalin@redhat.com of code extracted from Owl's package