etcupdate: Prefer POSIX -depth to BSD -d

This is in preparation for building an etcupdate tree on non-FreeBSD
when building release images. The -d option is documented as a
BSD-specific equivalent to the POSIX -depth primary. Whilst GNU find
sort of accepts it in an attempt to be compatible, it still doesn't
permit it coming before the paths, unlike BSD find, and prints a
deprecation warning either way. Thus, use the equivalent POSIX -depth to
ensure it works correctly and without warning everywhere.

Reviewed by: jhb
Obtained from: CheriBSD
Differential Revision: https://reviews.freebsd.org/D35856