BUG/MEDIUM: ssl: 'tune.ssl.default-dh-param' value ignored with openssl > 1.1.1

BUG/MEDIUM: ssl: 'tune.ssl.default-dh-param' value ignored with openssl > 1.1.1

If openssl 1.1.1 is used, c2aae74f0 commit mistakenly enables DH automatic
feature from openssl instead of ECDH automatic feature. There is no impact for
the ECDH one because the feature is always enabled for that version. But doing
this, the 'tune.ssl.default-dh-param' was completely ignored for DH parameters.

This patch fix the bug calling 'SSL_CTX_set_ecdh_auto' instead of
'SSL_CTX_set_dh_auto'.

Currently some users may use a 2048 DH bits parameter, thinking they're using a
1024 bits one. Doing this, they may experience performance issue on light hardware.

This patch warns the user if haproxy fails to configure the given DH parameter.
In this case and if openssl version is > 1.1.0, haproxy will let openssl to
automatically choose a default DH parameter. For other openssl versions, the DH
ciphers won't be usable.

A commonly case of failure is due to the security level of openssl.cnf
which could refuse a 1024 bits DH parameter for a 2048 bits key:

 $ cat /etc/ssl/openssl.cnf
 ...

 [system_default_sect]
 MinProtocol = TLSv1
 CipherString = DEFAULT@SECLEVEL=2

This should be backport into any branch containing the commit c2aae74f0.
It requires all or part of the previous CLEANUP series.

This addresses github issue #324.