stand: Use devformat instead of disk_devfmt

Use devformat instead of disk_devfmt. This allows us to avoid knowing
the details of the device that's underneath us. Remove disk.h include
and the -I${LDRSRC} from the build of ufs.c since they are no longer
needed.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D35922

[ minor tweaks due to earlier UFS MFC ]

(cherry picked from commit 0b3a4a588fb6b30d923686828dab4685b94ec1ea)