Mk/bsd.ports.mk: Add suppport for WWW in Makefiles

Expect the project website or other relevant URL of a port to be
specified in a WWW macro in its Makefile.

This information used to be available in WWW: lines at the end of
each port's pkg-descr file. By moving it into the Makefile, this
value is easier to access, verify, and maintain.

A WWW: line is added to the "desc" element of package manifests
based on the WWW macro value by the create-manifest.sh script.
This restores the previous contents of this line in the manifest
(as e.g. expected by the Freshports website).

The ports-mgmt/portlint port has been updated in commit 9800743f0
(version 2.19.13) to support the planned introduction of WWW in
port Makefiles.

Approved by: portmgr (tcberner)
Differential Revision: https://reviews.freebsd.org/D36434