use it in places explicit_bzero() used to be called.
The kore_mem_zero() is a best effort to try and let the compiler
not optimize the code away. Highly platform dependent.
This work moves all TLS / crypto related code into a tls_openssl.c
file and adds a tls_none.c which contains just stubs.
Allows compilation of Kore with TLS_BACKEND=none to remove building
against OpenSSL.
Also adds code for SHA1/SHA2 taken from openssh-portable so we don't
depend on those being present anymore in libcrypto.