www/varnish4: Update to 4.1.0

• Add varnish and varnishlog UIDs
• Add varnish GID
• varnishd, varnishncsa, and varnishlog no longer run as root

Users upgrading may see permissions errors on start due to change in
uid running the processes. Please see the UPDATING entry for more
details.

Changelog: https://github.com/varnish/Varnish-Cache/blob/varnish-4.1.0/doc/sphinx/whats-new/changes.rst

Differential Revision: https://reviews.freebsd.org/D3878