xz: Move sandboxing code to sandbox.c and improve Landlock sandbox.

xz: Move sandboxing code to sandbox.c and improve Landlock sandbox.

Landlock is now always used just like pledge(2) is: first in more
permissive mode and later (under certain common conditions) in
a strict mode that doesn't allow opening more files.

I put pledge(2) first in sandbox.c because it's the simplest API
to use and still somewhat fine-grained for basic applications.
So it's the simplest thing to understand for anyone reading sandbox.c.