Allow secret modification in exec-file mode

We have a case where a program needs to modify the secrets.

So the existing workflow is to make the program behave like an editor, and execute sops like:

EDITOR=path/to/tool sops path/to/encrypted_secret

This approach works for most of the case, however, we cannot pass arguments to the tool unless using a wrapper script for the conversion.

On the other hand, exec-file mode provides a much better interface for interacting with external tools. Which even uses FIFO device to avoid putting the secret on disk. It would be nice to make the FIFO bi-directional, so that the program could optionally write the modified content back to the device.

sops exec-file --edit path/to/encrypted_secert 'path/to/tool -file={} -other-flag'

Start by tracing the exec-file mode entry point and its FIFO handling. Check how the current one-way device is created and consumed, then determine how modified content could be written back without placing the secret on disk. Done means the shown command can pass arguments to an external tool and receive its modified secret content.