Add TLS-aware I/O paths

[quonic]

I was using this to integrate Twitch's EventSub in to my application and I fixed a few problems along the way. I figured contributing back might be of some help to others.

Unify platform error handling

• Replace raw errno usage with shared error helper procs
• Normalize invalid, refused, reset, would-block, and support errors

Add TLS-aware client transport

• Send HTTP requests through TLS sockets when enabled
• Add SNI-aware OpenSSL socket setup and TLS config ownership fixes
• Improve request serialization and response parsing for multi-value headers

Support TLS connections

• Use TLS reads and writes during WebSocket handshake and frame I/O
• Store TLS socket on Conn and close it correctly
• Improve ping, pong, close, and read-state handling