/dev/sequencer retiring

From: Christos Margiolis <christos_at_freebsd.org>
Date: Wed, 04 Jun 2025 16:20:57 UTC
As part of a series of MIDI patches and cleanups, I (along with adrian@
mostly) decided to retire /dev/sequencer [1] on 15.0, for the reasons
explained in the commit message. I've also pushed a deprecation notice
on main and stable/14.

Christos

[1] https://reviews.freebsd.org/D50605
[2] https://cgit.freebsd.org/src/commit/?id=799105f53a9647f86dbf08456260d5850851b6f9