sysutils/s6: Update to 2.7.2.1

While here, pet portlint:

• Fix COMMENT
• Fix order of variables

Changelog:
http://git.skarnet.org/cgi-bin/cgit.cgi/s6/tree/NEWS

PR: 232052
Submitted by: crest@rlwinm.de (maintainer)
Reported by: dewayne@heuristicsystems.com.au
Reviewed by: krion
Approved by: krion (mentor)
Differential Revision: https://reviews.freebsd.org/D17568