Improve host key verification

[jjakob]

Currently host key verification is done by checking if the host key fingerprint matches the expected one. This has a flaw in that it assumes that the server host key type will always be the one we are expecting. Servers can have multiple different host keys of different types. The host key selection is done based on the SSH protocol specification, from RFC4253 section 7.1:

      server_host_key_algorithms
         A name-list of the algorithms supported for the server host
         key.  The server lists the algorithms for which it has host
         keys; the client lists the algorithms that it is willing to
         accept.  There MAY be multiple host keys for a host, possibly
         with different algorithms.

         Some host keys may not support both signatures and encryption
         (this can be determined from the algorithm), and thus not all
         host keys are valid for all key exchange methods.

         Algorithm selection depends on whether the chosen key exchange
         algorithm requires a signature or an encryption-capable host
         key.  It MUST be possible to determine this from the public key
         algorithm name.  The first algorithm on the client's name-list
         that satisfies the requirements and is also supported by the
         server MUST be chosen.  If there is no such algorithm, both
         sides MUST disconnect.

This means that the client can choose which host key will be selected based on the sorted list of host key algorithms it sends to the server. This is also described here: https://www.bitvise.com/ssh-server-guide-host-keys

crypto/ssh has a way to set this list in ClientConfig:

	// HostKeyAlgorithms lists the key types that the client will
	// accept from the server as host key, in order of
	// preference. If empty, a reasonable default is used. Any
	// string returned from PublicKey.Type method may be used, or
	// any of the CertAlgoXxxx and KeyAlgoXxxx constants.
	HostKeyAlgorithms []string

easyssh doesn't set this list so it relies on crypto/ssh defaults which may change in the future. This means a crypto/ssh version update may break existing programs that use easyssh with fingerprint verification.

An easy improvement would be to allow specifying this list via MakeConfig. "Any string returned from PublicKey.Type method may be used", these are probably constant. Thus the host key type would always be guaranteed to match the key fingerprint.

Another improvement would be to be able to use func FixedHostKey as HostKeyCallback to allow specifying the full host public key to verify, not just the fingerprint, which would also be more secure.