usr_bin/grml-exec-wrapper: Fix a couple of shellcheck warnings

usr_bin/grml-exec-wrapper: Fix a couple of shellcheck warnings

- SC2059: Don't use variables in the printf format string.
- SC2086: Double quote to prevent globbing and word splitting.
- SC2166: Prefer [ p ] || [ q ] as [ p -o q ] is not well defined.

Please note that SC2059 causes SC1117 (Backslash is literal in "\n".
Prefer explicit escaping: "\\n") in shellcheck 0.5.0 an below.

This check was retired due to noise in koalaman/shellcheck@d8a32da07.