Set list of offered SSH authentication methods.

Set list of offered SSH authentication methods.

Make the SSH authentication methods used by the server configurable,
so that for example password authentication can be turned off.

For this, a `git.sshAuthenticationMethods` setting is added which is a space
separated list of authentication method names. Only the methods listed will
be enabled in the server.
This is modeled after the option of the same name from sshd_config, but it
does not offer listing multiple required methods. It leaves the door open,
though, for a later extension to support such a multi-factor authentication.

Since this also includes Kerberos authentication with GSS API, this obsoletes
the `git.sshWithKrb5` property. The latter is removed. Instead, to enable
Kerberos5 authentication, add the method name `gssapi-with-mic` to the
authentication methods list.