Refactor script based on ShellCheck suggestion

[PeterDaveHello]

Refactor the n script, based on my Shell Script experience and ShellCheck suggestion.

[shadowspawn]

I initially thought this might be a wild style change request, but when I looked into ShellCheck I was impressed by the depth and accuracy of the advice. Thanks @PeterDaveHello.

I am willing to do a formal review of the PR if helpful to maintainers.

[PeterDaveHello]

Any chance to move forward here, it started to have conflicts here, I'd like to resolve it if we can do something here ;) Thanks.

[shadowspawn]

I have recently been added as a maintainer. I am interested in making these changes, but do not want to take it as a Pull Request (sorry). I would like to work through the changes suggested by ShellCheck myself so I can check each change and do it incrementally. (I have done this once already on my own fork of n, inspired by this Pull Request.)

I don't want you to miss out on getting credited as a contributor though, if that matters to you. You could submit a new Pull Request with a single or small change, and could say in the commit message it is a small example of wider changes recommended?

[PeterDaveHello]

@shadowspawn changes are separated now, should be good enough?

[shadowspawn]

I have cherry-picked all but one of the commits into develop branch for next release.

(I left out "Declare and assign separately to avoid masking return values" as not currently testing the return values.)

[PeterDaveHello]

Thanks!

[shadowspawn]

Shipped in v4.0.0

Thank you for your contributions.