prepare-data-dir leaves files with a second name to their owner (closes #143)

A hard link in /data shares its owner with a file that can be anywhere
on the same filesystem, so chowning it gave that file to the netwatch
user. A regular file with more than one name now keeps its owner and
its path is logged.

Owners are now set on the opened directory or file after checking it,
as chmodDir sets modes, so a hard link swapped in afterwards is caught.
Symbolic links and other files that are neither directories nor
regular files are no longer given away: they cannot be opened safely,
and setting their owner by name could reach a hard link swapped in.

Model: opus-5-5

bin/entrypoint.sh

@@ -66,10 +66,11 @@ done > /etc/nginx/trusted-proxies.conf
 # netwatch-server keeps its report files in DATA_DIR, on the /data
 # volume, which may be a host directory owned by root or by another
 # uid. Here, as root, netwatch-server prepare-data-dir creates DATA_DIR
-# and gives /data and everything in it to the netwatch user, so the
-# host directory needs no preparing. It stops the start, naming
-# DATA_DIR, unless DATA_DIR is /data or a path below it, and it acts on
-# nothing outside /data, whatever symbolic links it meets there.
+# and gives /data and the directories and regular files in it to the
+# netwatch user, so the host directory needs no preparing. It stops the
+# start, naming DATA_DIR, unless DATA_DIR is /data or a path below it,
+# and it acts on nothing outside /data, whatever symbolic or hard links
+# it meets there.
 export DATA_DIR="${DATA_DIR:-/data/reports}"
 netwatch-server prepare-data-dir "$DATA_DIR" || exit 1