[exim] tls_eccurve = auto (Was: Exim 4.88 RC5 uploaded)

Hello Torsten,

now tls_eccurve = auto is the default and should work in any OpenSSL
version. For OpenSSL < 1.0.2 it falls back to prime256v1. For more
recent OpenSSL versions it uses the libraries auto-selection.

You're free to override the tls_eccurve parameter with any string, the
library understands as a valid ec curve parameter.

Please, feel free to test my patch and to verify the documentation:

    git://git.exim.org/~heiko/exim.git

branch hs-tls-eccurve-interop. It's based on the latest master.
If there are no complaints, I'll push it into the official repository.

    Best regards from Dresden/Germany
    Viele Grüße aus Dresden
    Heiko Schlittermann
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