Fix C port allocation for multiple concurrent devices

[Daniiiil1]

Summary

• reuse only closed C port slots instead of overwriting an active serial handle/file descriptor
• apply the same correction to the Windows, Linux, and macOS C backends
• preserve the existing behavior for a previously registered device name

Fixes #670.

Testing

• configured and built the standalone C SDK with CMake on macOS
• opened two independent pseudo-terminal devices through the C API and verified that both ports opened successfully with distinct indices (0 and 1)
• git diff --check