SnapRAID v15.1

• Add synthetic device IDs on Linux, allowing SnapRAID Daemon to
  preserve history for devices without unique hardware IDs while
  they remain connected within the same boot.
• Fix a failed internal inconsistency check when '--gui-touch-before'
  is used before the first sync when no content file exists yet.
  Also ensure that commands that can save the content file ('touch',
  'scrub', 'rehash', 'rewrite') fail with a clear error message if the
  array has not been synced yet. This fix is required by
  SnapRAID-Daemon.
• Make test and doc variables in Makefile.am compatible with BSD make,
  allowing builds, tests, and documentation generation on BSD systems
  without requiring GNU make.