Introduce USES=varnish
You can now set the default version of Varnish with DEFAULT_VERSIONS to
build Varnish and its modules against your desired Varnish release
train.
The default version of Varnish throughout the ports tree will remain as
www/varnish4 until at least 2017Q1 when it will be changed in HEAD to be
www/varnish5. This should allow plenty of time for migration and for
stabilization of the changes in Varnish 5.x.
Submitted by: vanilla