rsync: improve robustness of scripts, fix non-interactive mode

rsync: improve robustness of scripts, fix non-interactive mode

POSIX shell:
- In non-interactive mode an infinite loop occurred unless the --yes
  argument was passed.
- `which` is a non-standard program, `command -v` is a POSIX standard
  shell builtin.
- `exec` prevents potential glitches when the script is updated by rsync
  while still running.

Windows batch:
- Variable assignments persisted after batch exit, potentially
  interfering with other batch execution.
- If copying failed the window closed before the error could be read.