Add support for `-A` (askpass) commandline switch

[allisonkarlitskaya]

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hi,

Cockpit uses sudo to gain root privileges for users performing actions via the web interface. We definitely need askpass support: we forward the prompt to the user, asking for the password, and passing it back. We can't do that with a pseudoterminal forwarded to the user because we also support caching the login password and using that.

See https://github.com/cockpit-project/cockpit/issues/22305 for the original report on our side.

Thanks!

Originally posted by @allisonkarlitskaya in #129