Fix mergemaster after r347638 and the master.passwd / group move

[brd]

Chase etcupdate build as well

[allanjude]

usr.sbin/mergemaster/mergemaster.sh

673–674

We need to check if this new path exists, and fail back to the old path, so that this can be used in say 12.1 for both upgrading to 12.2 or 13.0.

We'll then also need to MFC it to all supported branches.

[brd]

Add logic for etcupdate and mergemaster to handle the old and new location.

This should be MFCed to stable/12 and stable/11.