Code quality/refactor

[mgree]

We should:

• Unify error reporting and debug output #103
• Write style guide; normalize variable names, quoting, etc. #108 and Whitespace lint script #111 and Style fixes and typos #115 and Linter for tabs #116
• Clarify dependencies, distinguishing runtime from testing #100
• No exit when union_helper not found #99 (see also don't exit on union_helper not found #96)
• Run shellcheck in CI (mk 3) #97 (see also Run shellcheck on CI #72)
• Hide try-internal variables from inner script #114 (see also Hide internal variables #76)
• Nested mount support #67 (resolving Fail early when critical commands are missing #45)
• Refactor tests: each test is a file #117 (see Auto cleanup temporary files #66 for some initial directions)
• Check that versions in the program and manpage match in CI #105

[angelhof]

Great list!

[a1346054]

Consider using shfmt to unify the codestyle across the scripts, and perhaps enforcing it in CI.

Consider picking some shell styleguide for the project, such as https://google.github.io/styleguide/shellguide.html