dsnap-sync: introduce snapper config-types and config-postix

o snapper config-types
  enhance snapper config template which new fields
  CHILD_CONFIG=<true | false>  default: missing -> means false
  PARENT_CONFIG=<parent config name>
o Use the types for distinctable snapshot runs:
  disk2disk (1st stage snapshots  CHILD_CONFIG='false' or missing
  cloning   (2st stage snapshots  CHILD_CONFIG='true' or missing
o snapper config postfix
  to overcome snapper config name clashes, enable dsnap-sync with
  selectable postfix (default: .<hostname>)
  cloning a "home" config to remote host will not interfere with
  a "home" d2d-config on this remote host
o update the snapper template dsnap-sync

etc/snapper/config-templates/dsnap-sync

@@ -0,0 +1,61 @@
+###
+# snapper template for dsnap-sync handling
+###
+
+# subvolume to snapshot
+SUBVOLUME="/var/lib/dsnap-sync"
+
+# filesystem type
+FSTYPE="btrfs"
+
+
+# users and groups allowed to work with config
+ALLOW_USERS=""
+ALLOW_GROUPS="adm"
+
+# sync users and groups from ALLOW_USERS and ALLOW_GROUPS to .snapshots
+# directory
+SYNC_ACL="yes"
+
+
+# start comparing pre- and post-snapshot in background after creating
+# post-snapshot
+BACKGROUND_COMPARISON="yes"
+
+
+# run daily number cleanup
+NUMBER_CLEANUP="no"
+
+# limit for number cleanup
+NUMBER_MIN_AGE="1800"
+NUMBER_LIMIT="50"
+NUMBER_LIMIT_IMPORTANT="10"
+
+# "no": we will use systemd.timer
+TIMELINE_CREATE="no"
+
+# create cron based cleanup entries
+# "no": we will use systemd.timer
+TIMELINE_CLEANUP="no"
+
+# snap-sync: timeline settings
+TIMELINE_MIN_AGE="1800"
+TIMELINE_LIMIT_HOURLY="1"
+TIMELINE_LIMIT_DAILY="2"
+TIMELINE_LIMIT_MONTHLY="1"
+TIMELINE_LIMIT_YEARLY="1"
+
+
+# cleanup empty pre-post-pairs
+EMPTY_PRE_POST_CLEANUP="yes"
+
+# limits for empty pre-post-pair cleanup
+EMPTY_PRE_POST_MIN_AGE="1800"
+
+# uncomment to exclude this subvol when calling
+# snap-sync as timer unit
+# SNAP_SUNC_EXCLUDE="yes"
+
+# uncomment if your config will be used as snapshot-clone
+#CHILD_CONFIG=true
+#PARENT_CONFIG=<parent config name>