* refactor(install.sh): fix static analysis warnings

Clear all warnings and errors raised by shellcheck.net static analysis.

- Replace non-POSIX shell use of `$OSTYPE` by POSIX compliant check on `uname -o`.
- Move variables out of`printf` format string.
- Refactor/simplify string formatters for error and underline.
- Fix expansion of arguments to a single string `$*` rather than individual elements `$@` within the error and underline formatters.

* fix(uname): non-posix -o option

* fix(install.sh): non portable which

Replaced non-portable `which zsh` by portable `command -v zsh`