usr_sbin/dirvish-setup: Fix some shellcheck warnings

usr_sbin/dirvish-setup: Fix some shellcheck warnings

- Ignore SC1091: Not following: /etc/grml/script-functions was not specified as input (see shellcheck -x).
- Ignore SC1091: Not following: /etc/grml/lsb-functions was not specified as input (see shellcheck -x).
- SC2046: Quote this to prevent word splitting.
- SC2006: Use $(..) instead of legacy `..`.
- Ignore SC1090: Can't follow non-constant source. Use a directive to specify location.
- SC2046: Quote this to prevent word splitting.
- SC2006: Use $(..) instead of legacy `..`.
- SC2181: Check exit code directly with e.g. 'if mycmd;', not indirectly with $?.
- SC2046: Quote this to prevent word splitting.
- SC2181: Check exit code directly with e.g. 'if mycmd;', not indirectly with $?.
- SC2086: Double quote to prevent globbing and word splitting.
- SC2181: Check exit code directly with e.g. 'if mycmd;', not indirectly with $?.