By default sshd will use its default built-in list, which matches
the one of openssh (as far as the algorithms exist in sshd at all).
But it doesn't handle HostKeyAlgorithms from the ssh config at all.
Implement this as in openssh, including the '+' and '-' modifiers
and reordering the default if there are known host keys for a
server already.
Add tests for the reordering.
Also use a more robust reader for the known hosts file. The default
aborts on the first error.
Bug: 520927
Change-Id: Ib1684440bfe2e96140536aa1a93c4bd4a0d35916
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>
ec1116627f