netcmd: Determine which files are to be copied for an offline domain backup

netcmd: Determine which files are to be copied for an offline domain backup

The old behaviour attempted to check for and remove files with duplicate
names, but did not do so due to a bug, and would have left undetermined
which files were given priority when duplicate filenames were present.
Now when hardlinks are present, only one instance of each file is
chosen, with files in the private directory having priority. If one
backup dir is nested inside another, the files contained in the nested
directory are only added once. Additionally, the BIND DNS database is
omitted from the backup.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14027

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
(cherry picked from commit 3723148e7aa7e6d4a48a1a38112f121f52b6ee6f)