doc: fix SSH_BIND_OPTIONS_RSAKEY typo
options: add SSH_BIND_OPTIONS_ECDSAKEY
pki: Fix ability to use ECDSA host keys

- Ensure ecdsa_nid is copied into duplicated private keys
- Use SHA-2 for sessionid generation with ECDSA keys (was SHA-1)

Signed-off-by: Alan Dunn <amdunn@gmail.com>
kex: enable more ECDSA hostkey algos