multipath-tcp / mptcpd / 19820416316

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plugin: expose subflow error to plugins (#331)

* plugin: expose subflow error to plugins

Add error parameter to subflow_closed callback to allow plugins
to identify the reason for subflow closure, such as network errors.

The error value corresponds to the MPTCP_ATTR_ERROR attribute from
the kernel, which equals sk_err from struct sock. If the error
attribute is not present in the netlink message, it defaults to 0.

Fixes #330

* Improve error parameter documentation and test coverage

- Add non-zero test value (110) for test_error_4 to verify different values
- Enhance error parameter documentation to clarify it's an errno from sk_err
- Update both public and private plugin headers with detailed description

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