Detect unshared test namespace support

[huangminghuang]

Summary

• replace the GitHub Actions-only unshared test toggle with CMake capability detection
• add SYSIO_UNSHARED_TESTS=AUTO|ON|OFF to control network namespace isolation for tests
• document the configure-time option in README.md

Validation

• ran the Python unshare probe logic locally; this host reports namespaces unavailable with EPERM, so AUTO would disable --unshared here
• did not run a full CMake test configure/build