MINOR: ssl: Add 'ssl-provider' global option

MINOR: ssl: Add 'ssl-provider' global option

When HAProxy is linked to an OpenSSLv3 library, this option can be used
to load a provider during init. You can specify multiple ssl-provider
options, which will be loaded in the order they appear. This does not
prevent OpenSSL from parsing its own configuration file in which some
other providers might be specified.
A linked list of the providers loaded from the configuration file is
kept so that all those providers can be unloaded during cleanup. The
providers loaded directly by OpenSSL will be freed by OpenSSL.