Fix signature algorithm selection logic for UpdateHostkeys on the

Fix signature algorithm selection logic for UpdateHostkeys on the
server side. The previous code tried to prefer RSA/SHA2 for hostkey
proofs of RSA keys, but missed some cases. This will use RSA/SHA2
signatures for RSA keys if the client proposed these algorithms in
initial KEX. bz3375

Mostly by Dmitry Belyavskiy with some tweaks by me.

ok markus@