Skip remount async when systemd is used (issue1097)

usr/share/rear/restore/default/050_remount_async.sh

@@ -5,10 +5,19 @@
 # via finalize/default/900_remount_sync.sh
 # see also https://github.com/rear/rear/issues/1097
 #
+# Remounting with async option is not needed when systemd is used because
+# when systemd is used remounting with sync option is skipped in a preceding
+# recover WORKFLOW via finalize/default/900_remount_sync.sh and to aviod
+# needless operations remounting with async option is also skipped here
+# cf. https://github.com/rear/rear/issues/1097
 
 # Skip if not restoreonly WORKFLOW:
 test "restoreonly" = "$WORKFLOW" || return 0
 
+# Skip if systemd is used
+# systemctl gets copied into the recovery system as /bin/systemctl:
+test -x /bin/systemctl && return 0
+
 while read mountpoint device mountby filesystem junk ; do
     if ! mount -o remount,async "${device}" $TARGET_FS_ROOT"$mountpoint" ; then
         LogPrint "Remount async of '${device}' failed which can result very slow restore"