stand: Create DEV_NAMLEN for the length of a device name

Rather than hard coding 8 for the device name length, create a #define for it.

Sponsored by: Netflix
Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D35910

(cherry picked from commit 4932a6e41d1fc3fe474358501d5933e783066bdc)