Add an environment variable to control stdio mode

[tgauth]

Prerequisites

• Write a descriptive title.
• Make sure you are able to repro it on the latest version
• Search the existing issues.

Steps to reproduce

see PowerShell/openssh-portable#759

Expected behavior

support different stdio modes via OPENSSH_STDIO_MODE env variable

Actual behavior

current behavior defaults to NONSOCK_SYNC_FD stdio mode only

Error details

No response

Environment data

N/A

Version

9.8.1.0

Visuals

No response

[tgauth]

see https://github.com/PowerShell/openssh-portable/pull/759/files