- main loop will now gracefully exit when rsync is not found or
  returns anything except 0 or 24.
  - pid unlock works now in this case
  - probably we will need to handle other rsync return codes like 24

  - otherwise, if reusing an incomplete snapshot, the rename operation
  would fail because old and new name are the same
  - this would only happen if rsync runs less than a second, so normally
  in some error case
  - the 1 second sleep makes sense because 1 second is the resolution of
  the snapshot names

  - if it is ErrHelp, return 0 and exit
  - before "snaprd subcmd -h" returned a non-zero value

  - basically move all of main() into another function and make that
  deal with errors
  - this enables defers to run properly on exit
  - needs a lot more fixes deeper in the code to never use os.Exit() and
  such

  - most other situation where log.Fatal() is used should be converted
  to this style
  - overall goal is to make the deferred pid unlock work

  - also note how to report issues

  - use %v for error values

  - remove duplicate code

  - needs subcmdList() to take a clock argument
  - should detect problems like fbd75161

This reverts commit 5596cf7e.

Actually, we want the shadowing here: For each iteration we need
the snapshot list to be reset, otherwise the listing will be wrong
(empty) for all intervals but the first listed.