sound: Untangle dsp_cdevs[] and dsp_unit2name() confusion

Before de8c0d15a64fa ("sound: Get rid of snd_clone and use
DEVFS_CDEVPRIV(9)"), sound(4) would create one device for each allocated
channel. The device names would be chosen from dsp_cdevs[], and created
with dsp_unit2name(). Additionally, dsp_cdevs[] was also used to match
these devices names, as well as OSSv4 aliases in dsp_clone().

Since sound(4) does not create separate devices for each channel
anymore, the meaning and use dsp_cdevs[] has changed. Part of it no
longer corresponds to devices at all, but instead is used to create
channel names, and another part is used to match only OSSv4 aliases in
dsp_clone().

To address this confusion, separate dsp_cdevs[] into a dsp_aliases[]
array, and move dsp_unit2name() to pcm/channel.c and rename it to
chn_mkname().

While here, get rid of the SND_DEV_DSPHW_* channel types, and simply use
the existing PCMDIR_* constants as the channel types. There is no need
to duplicate the same meaning twice.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D47199

(cherry picked from commit 802c78f5194e4524faa30ea57adbf00f28fc72c6)
(cherry picked from commit b1bb6934bb8774dd96be76d88e6824e49b613549)