Add a request structure and make the implementation use it.

This allows compatibility translation to take place on the stack
(md_ioctl is too big) and is more suitable as a public interface within
the kernel than the kern_ioctl interface.

Except for the initialization of the md_req from the md_ioctl
(including detection of kernel md_file pointers) and the updating
of the md_ioctl prior to return, this is a mechanical replacment
of md_ioctl and mdio with md_req and mdr.

Reviewed by: markj, cem, kib (assorted versions)
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D14704