Various incremental improvements

[jnd-au]

This PR addresses multiple issues:

1. Minor code style improvements.
2. Unit-test coverage raised from 84.5% to 93.2% (including main.go coverage raised from 31.6% to 76.5%).
3. Fix Exit code for empty -list search #70
4. Fix Cannot show UNIX epoch with -when 0 #71
5. Fix main.go lacks sufficient unit test coverage #72

This includes feature-support for date-times like tz -when 2006-01-02T15:04:05-07:00.

Internally, main.go tests are supported through the new files os_wrapper.go and os_wrapper_test.go which provide our own os:

1. Capture & test exit codes without exiting.
2. Capture & test stderr messages instead of printing to console.
3. Test flag CLI argument parsing.
4. Test error-handling code paths.
5. Clean env vars for each test.
6. Simulate home dir scenarios.
7. Virtual test file I/O in-memory

The residual warts are:

1. Need to add return after os.Exit to support unit testing (in unit-test mode, our os.Exit records the exit code without exiting, so the function needs to return instead).
2. Need to access os.Stdout() as an interface function instead of a package field.

[oz]

I'm grateful for the contributions, but please don't make one PR for shopping-list issues that you also created yourself.

[jnd-au]

FYI all the commits are interrelated with providing the missing test coverage, which is why I bundled these changes after the other PRs.