Major refactor - strict mode and functions

[robinbowes]

Two main changes here:

1. Refactor to use bash "strict" mode.
2. Put all code in functions, with a main() entry-point.

(1) means you don't need to test every command for success - the script exits if a command fails
(2) is my preferred coding style as it it makes it very clear what the script is doing

I also added a fail() utility function.

Fixes #14

[calmh]

You can add a (fixes #14) to this one 👍

I assume you've done what testing you can manually on this?

[robinbowes]

This should now be good for testing.

I've done some testing myself (upgraded my smartoOS node) but it probably needs more.