Add libcurl-style Happy Eyeballs IPv6/IPv4 fallback

[koranyellow]

This patch introduces an RFC8305 / libcurl-inspired Happy Eyeballs implementation to the Mosquitto client library. It performs dual-stack connection racing with a delayed IPv4 fallback, using fully non-blocking sockets and selecting the first socket that successfully completes the TCP handshake.

It preserves existing Mosquitto APIs and does not alter ABI.

Close #574

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