Port h_hash and t_sha2 to FreeBSD

t_sha2 contains dirty copy-paste hacks that need to be fixed with the openssh
OpenBSD compat layer

Submitted by: pho


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273952 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f