Remove the CDIOCREADSUBCHANNEL_SYSSPACE ioctl.

This was added for emulation of Linux's CDROMSUBCHNL, but allows
users with read access to a cd(4) device to overwrite kernel memory
provided that the driver detects some media present.

Reimplement CDROMSUBCHNL by bouncing the data from CDIOCREADSUBCHANNEL
through the linux_cdrom_subchnl structure passed from userspace.

admbugs: 768
Reported by: Alex Fortune
Security: CVE-2019-5602
Security: FreeBSD-SA-19:11.cd_ioctl