main/varnish: upgrade to 6.4.0
https://github.com/varnishcache/varnish-cache/blob/6.4/doc/changes.rst#varnish-cache-640-2019-09-16
Changed libexecinfo to libunwind as changes said but reverted as the library not available for a lot of arches
https://github.com/varnishcache/varnish-cache/blob/6.4/doc/changes.rst#varnish-cache-640-2019-09-16
Changed libexecinfo to libunwind as changes said but reverted as the library not available for a lot of arches