netrc: remember and check filename loaded

[icing]

Remember the filename of a loaded netrc file to detect changed configurations in a reused easy handle.

[Copilot]

Pull request overview

This PR updates curl’s netrc caching so a reused easy handle can detect when a different netrc filename is being scanned and (re)load the file accordingly, rather than reusing previously loaded contents blindly.

Changes:

• Clears the in-memory netrc buffer before (re)loading a netrc file.
• Tracks the loaded netrc filename in store_netrc and reloads when the requested filename differs.
• Initializes and frees the stored filename in Curl_netrc_init() / Curl_netrc_cleanup().

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.