Add support for remote sudoers

[aplopez]

This PR addresses #1421.

I include four commits in the PR:

• The support for the @socket directive (done by @squell)
• The logics for reading the sudoers rules from the socket, and
• A test for this case.
• The documentation for @socket.

Some interesting points.

Limiting the "include" level

I replaced the old safety_count parameter (a u8 counter limiting the number of included files) by the IncludeState enum that includes this counter when inclusions are allowed, but also allows to forbid them. This was needed to prevent new inclusions from a socket even if the limit was not reached.

Test

I created an e2e test because I needed to create a socket on the filesystem. Otherwise I would have created a unit test.

Protocol

As we discussed some time ago, to make things simpler, the rules must be provided as text through the socket.

Use of AI

The use of AI was limited to:

• Help me learn a few things that I had never done before in Rust, such as opening a socket.
• Understand how some pieces of the existing code work, in particular the test framework.
• Identify a few C-style pieces of code that I produced (habits!) and get suggestions to make them more rustacean.

All the code was produced by me.

[aplopez]

I just updated the documentation with the @socket directive.

[bjorn3]

Looks pretty good. Just a couple of small nits.

[bjorn3]

Thanks for implementing this!

[squell]

🙏 Thanks everybody for the great and patient collaboration.

[aplopez]

Thanks for the guidance and patience.