Extend GSSAPI configuration support to ssh_config

[wzzrd]

Previously, the ssh_gssapi_support variable only toggled the GSSAPI
settings in sshd_config.

Through this change, setting ssh_gssapi_support to true also enables
support in ssh_config.

It enables both authentication and credential delegation.

[wzzrd]

I find that enabling support in the SSH client for both authentication and delegation is appropriate, considering the broardly named variable (ssh_gssapi_support).

Alternatively, the original variable could be renamed sshd_gssapi_support (for clarity), and I could introduce new variables for the authentication and delegation settings in ssh_config valled ssh_gssapi_auth and ssh_gssapi_delegation. Significant downside is changing current behaviour.

Let me know what you think is best.

[schurzi]

Hey @wzzrd thanks for noticing this. As you already observed, renaming of variables is a bit problematic because of backward compatibility. So I would not like to do this here. We should do stuff like this in a later major release, then we will untangle all variables in one change.

I am currently a bit concerned about directly enabling delegation, because I lack understanding of it and I see parallels to ssh agent forwarding, which might be a security concern (https://documentation.help/PuTTY/config-ssh-auth-gssapi-delegation.html). Because I don't understand it well enough to make an informed descision directly, I prefer to set this to no as a default. I also like the variable name ssh_gssapi_delegation for this.

For enabling GSSAPI support in the client you can keep ssh_gssapi_support if the variable manages configuration for both the client and the server this is consistent with the other variables.

[wzzrd]

Updated the PR

[schurzi]

thank you for contributing :)