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Add a “skip_dsn” option to g_part's bootcode verb to prevent g_part_mbr

Description

Add a “skip_dsn” option to g_part's bootcode verb to prevent g_part_mbr
from setting the volume serial number. This unbreaks older boot blocks
that don't support serial numbers, and allows boot0cfg to set the serial
number itself if requested by the user.

Submitted by: lev@, yuripv@
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D17386