Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r253986:
Fix formatting warning.

MFC r239070:
Add simple test program that uses the partition tables handling code.
It is useful to test and debug how boot loader handles partition tables
metadata.

MFC r239087:
Add to the debug output the offset from the parent partitioning scheme.

Add simple test program that uses the partition tables handling code.
It is useful to test and debug how boot loader handles partition tables
metadata.