Shellcheck emits warnings for `install.sh` and `uninstall.sh`

[dylannorthrup]

shellcheck is pedantic, so there are things that work which it still complains about. I created #1 to silence those errors and am creating a corresponding issue (in case that's needed).

Please treat the changes as suggestions and include or ignore as you like.

Also, thanks for the single-user install script! It's exactly what I needed!