Call cap_enter.

Once in capability mode, access to global name spaces, such as file system or IPC name spaces, is prevented by the operating system. A program can call this after opening any directories, files or network sockets that it will need, to prevent accidental access to other resources.

The standard environment directories Eio.Stdenv.fs and Eio.Stdenv.cwd cannot be used after calling this, but directories opened from them via Eio.Path.with_subtree will continue to work.