kex: implement curve25519-sha256@libssh.org
doc: Documentation of curve25519-sha256@libssh.org
callbacks: add support for auth_none_function
session: Try the ecdsa default key first.
channel: Fix packets termination timeout in global_request().

BUG: https://red.libssh.org/issues/126
submodule compile fixes
Merge branch 'master' of http://git.libssh.org/projects/libssh