[BUG #7257] add watch-files/command/exec-on-startup config key

- also add recent changes to config schema
- renamed watch-files/exit-on-retcode to watch-files/command/exit-on-retcode
[BUG #7257] add support for watch-files/command/exec-on-startup

- also improved time stamp handling, as sometimes the changes did not result in
  a command execution