ssl_ecdh_curve is sometimes ignored in server blocks

[Avamander]

Consider a scenario when a single IP x.y.z.q has two server blocks. Both server blocks listen on the same port and support TLS. One of those blocks is marked default_server and handles the non-SNI requests.

If both blocks define ssl_ecdh_curve then it has zero effect on the non-default_server. This is done without warning.

One of the possible implications of this is that a more secure configuration is silently ignored. (I stumbled upon this when trying to enable post-quantum key exchange algorithms.)

Understandably nginx can't respect the directive in both blocks, but in that case the ignored one should throw a non-critical warning. Plus, it could be better-documented.