Make buildenv portable for bash compatibility

[MikeFultonDev]

This PR makes the buildenv file compatible with both bash (used on desktop systems) and UNIX System Services 'sh' (used on z/OS).

Changes

1. Added return statements to functions - Functions in buildenv now include explicit return statements, allowing the file to be sourced by bash without errors. Previously, the buildenv could only be sourced by z/OS USS 'sh'.

2. Added .zdx.json to .gitignore - The .zdx.json file contains user-specific configuration and should not be tracked in version control.

Benefits

• Enables desktop-based port development workflow using localzopen commands
• Allows developers to source buildenv on their local machines for testing
• Maintains full compatibility with z/OS USS 'sh'
• Improves developer experience for zopen port contributors

Testing

Verified that buildenv can now be sourced successfully on macOS with bash:

source ./buildenv  # No errors

[augmentcode]

🤖 Augment PR Summary

Summary: Updates buildenv so it can be sourced under both bash and z/OS USS sh by making the suite3270 symlink refresh idempotent and ensuring the empty hook functions return cleanly.
Also: Adds .zdx.json to .gitignore to avoid committing user-specific configuration.

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[augmentcode]

Review completed. 1 suggestion posted.

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[augmentcode]

ln -sfn relies on the non-POSIX -n flag, which may not be available on all ln implementations (potentially including some USS setups); if unsupported, sourcing/running buildenv will fail at this step.

Severity: medium

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