nginx: fix HTTP/3 reuseport duplicates

[Boubik]

Fix HTTP/3 reuseport duplicates

This PR fixes an Nginx config-generation issue introduced while adding optional HTTP/3 (QUIC) support in nginx: add optional HTTP/3 #5071.
When multiple server blocks ended up emitting reuseport for the same listen (same address:port), Nginx failed to start.

Reference PR: nginx: add optional HTTP/3 #5071 (#5071)

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Problem

With HTTP/3 enabled on more than one server for the same address:port, the generated configuration could contain reuseport multiple times for the same socket.
reuseport is a socket option and must be applied consistently for a given address:port. When duplicated across multiple server blocks for the same listener, Nginx reports an error and refuses to start.

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Fix

We now emit reuseport exactly once per address:port for HTTP/3:

• The first generated HTTP/3 listen for a given address:port gets: quic reuseport
• Any additional HTTP/3 listen directives for the same address:port get: quic (without reuseport)

This guarantees that the socket option is not duplicated and prevents Nginx from failing due to conflicting listen parameters.

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Testing

Tested on a local OPNsense instance with multiple server blocks sharing the same address:port and HTTP/3 enabled on more than one of them.
If you have a different setup (especially more complex listener/server combinations), please give it a quick try — there may still be edge cases I did not cover.