Update Subsonic from 5.3 to 6.0 and update license accordinly.

Former maintainer has requested to drop maintainership also, so put it in
ports@FreeBSD.org's pool.

PR: 213298
Submitted by: Allison Nicole Reid <root at cooltrainer org>
Approved by: adamw (Phabricator), jpaetzel (IRC)
Differential Revision: https://reviews.freebsd.org/D11445