MINOR: ssl: ssl-load-extra-files configure loading of files

MINOR: ssl: ssl-load-extra-files configure loading of files

This new setting in the global section alters the way HAProxy will look
for unspecified files (.ocsp, .sctl, .issuer, bundles) during the
loading of the SSL certificates.

By default, HAProxy discovers automatically a lot of files not specified
in the configuration, and you may want to disable this behavior if you
want to optimize the startup time.

This patch sets flags in global_ssl.extra_files and then check them
before trying to load an extra file.