Improve backwards compat with 1.0.2 for ECDHParameters

In 1.0.2 you could configure automatic ecdh params by using the
ECDHParameters config directive and setting it to the value
"+Automatic" or just "Automatic". This is no longer required in 1.1.0+
but we still recognise the "+Automatic" keyword for backwards compatibility.
However we did not recognise just "Automatic" without the leading "+" which
is equally valid. This commit fixes that omission.

Fixes #4113

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6036)