Initial commit; base scripting and systemd user event code for borg based backup

etc/excludes.example

@@ -0,0 +1,9 @@
+# Optional Borg exclude patterns, one per line.
+# The backup runs from inside SOURCE_DIR, so patterns are relative.
+#
+# Examples:
+# cache directories:
+# */.cache
+#
+# thumbnails:
+# */thumbnails