Support rootpw option

[jplatte]

Describe the feature you'd like see implemented in sudo-rs

sudo has an option rootpw that I've been using as Defaults rootpw (I'm sure you can somehow also use it in a more granular way). This makes sudo prompt for the root password rather than the user's own password (which is also reflected in the prompt).

What problem can be solved with this feature?

My previous workplace had a policy of requiring separate user / system administration passwords, and this was one of the things that needed to be configured for it on Linux. I'm a bit skeptical about the real-world benefits of this (c.f. https://xkcd.com/1200/), but it doesn't hurt either and I've adjusted to it.