[key.c key.h readconf.c readconf.h ssh_config.5 sshconnect.c]
[dns.c dns.h README.dns ssh-keygen.1 ssh-keygen.c]
add experimental support for verifying hos keys using DNS as described
in draft-ietf-secsh-dns-xx.txt. more information in README.dns.
ok markus@ and henning@