Better handling of "stop" and "finish", and related changes.

Give info when exiting due to "finish" and "stop" files. Make
location of "finish", "stop" and "hold" files configurable.

* lysrdiff-settings: New script containing settings on "finish",
  "stop" and "hold" file location
* backup-repeatedly: Load new settings file. Use configurable file
  locations. Give message regarding cause of exit. When exiting before
  a backup run has been started, regard it as an error.
* backup-all: Load new settings file. Use configurable file locations.
* Makefile (install): Added lysrdiff-settings.