Implement sndstat nvlist-based enumeration ioctls.

These ioctl commands aim to provide easier ways for user space
applications to enumerate existing audio devices and the node they can
potentially use.

The exchange of device lists between user space and kernel is done on
nv(9). Some ioctl commands are added to /dev/sndstat node:

• SNDSTAT_REFRESH_DEVS
• SNDSTAT_GET_DEVS
• SNDSTAT_ADD_USER_DEVS
• SNDSTAT_FLUSH_USER_DEVS

Bump __FreeBSD_version to reflect the addition of the ioctls.

Sponsored by: The FreeBSD Foundation
Reviewed by: hselasky
Approved by: philip (mentor)
Differential Revision: https://reviews.freebsd.org/D26884