Fix errors in shell-scripts, make installation ask permission before modifying bashrc, make POSIX-compatible

[ELLIOTTCABLE]

Cleaned up the shell-scripts subsantially:

• Missing quotes were causing this to fail when installed on systems with spaces in the paths
• The install scripts now ask permission before modifying .bashrc, and provide instructions if that permission is denied
• All the scripts are now POSIX-compliant, and do not depend on bash
• The README used curl ... | sh, which is slightly less safe than sh -c "$(curl ...)"r
• While at it, I cleaned up quite a few shellcheck warnings

[ELLIOTTCABLE]

In retrospect, I should have made separate PRs; apologies. Didn't realize how many things in the scripts I'd want to clean up once I'd started. 😅