Add OpenFile option to Config

[benhoyt]

Copying from @vyskocilm's original PR #291:

This allows user to provide a custom file opener instead of a default os.OpenFile, making goawk compatible with a newer standard library features like os.Root protecting against path traversal attacks or simulating a read-only filesystems.

Attached test cases for os.Root and a read-only filesystem.

TL;DR;

My motivation is my opensource project https://codeberg.org/gonix/, which is an user-space implementation of a unix scripting in Go. Somewhere goawk matches perfectly. Thanks for spending your time on it ❤️ The first application of my userspace scripting is a secure, yet shell scriptable, MCP server for agents https://codeberg.org/gonix/mcp.

At the moment it is secured by the fact most of available commands runs under Web Assembly and a shell layer mvdan.cc/sh/v3 allows one to define a proper handlers for os.OpenFile, os.Stat or os.Readdir. See

https://codeberg.org/gonix/mcp/src/commit/e5cff85075e6191e6415fa66435a4c73470cc978/internal/tools/shell_execute/tool.go#L159-L163

I kept the OpenFileFunc compatible with os.OpenFile and os package, however an another possibility to consider is a form with a context, mainly because a sh module uses it to pass various stuff like a working directory there.

// OpenFileFunc specifies a function used to open files instead of the default
// [os.OpenFile]. ctx is a context passed via [Interp.ExecContext], or [context.Background()]
// if [Interp.Exec] has been called.
OpenFileFunc func(ctx context.Context, name string, flag int, perm os.FileMode) (*os.File, error)

However it is up to your judgement.

BTW: an another task for an air-gaped goawk is to replace *exec.Command, but I kept this PR focused on a single topic. Additionally I am not entirely sure how to solve that one :-).