resources: file: Implement file attributes

Add owner which must be username or uid of the file owner, group which is
the group name or gid of the file, and mode which is the octal unix file
permissions.

Add separate implementation for Go 1.6 and lower.

test/shell/file-owner.sh

@@ -0,0 +1,24 @@
+#!/bin/bash -e
+# vim: noet:ts=8:sts=8:sw=8
+
+set -x
+
+if ! timeout 1s sudo -A true; then
+	echo "sudo disabled: not checking file owner and group"
+	exit
+fi
+
+# run till completion
+timeout --kill-after=15s 10s sudo -A ./mgmt run --yaml file-owner.yaml --converged-timeout=5 --no-watch --tmp-prefix &
+pid=$!
+wait $pid	# get exit status
+e=$?
+
+ls -l /tmp/mgmt
+
+test -e /tmp/mgmt/f1
+test -e /tmp/mgmt/f2
+test $(stat -c%U:%G /tmp/mgmt/f1) = root:root
+test $(stat -c%u:%g /tmp/mgmt/f2) = 1:2
+
+exit $e