Templates/Makefile: Modernize and extend

The template for a port Makefile can be used automatically by editors
like vim. It hasn't been updated since its introduction in 2014 though.

Modernize and extend it to meet current best practices:

• Replace PORTVERSION with DISTVERSION, which is now the preferred variable to use.
• Add the WWW variable which should be set for every port.
• Add the LICENSE and LICENSE_FILE variables which should also be set whenever possible.

Approved by: bapt
Differential Revision: https://reviews.freebsd.org/D42155