Simplify Watson main script and trying to make it more POSIXy

[pablojimpas]

I was recently learning some syntax differences between POSIX sh and bash while reading pure-sh-bible and pure-bash-bible. So I thought it will be great to put some things in practice contributing to Watson.

It's an initial work in progress to get the script fully POSIX compliant but I already consider some of the syntax changes an improvement.

I'm also using https://github.com/koalaman/shellcheck to fix some things along the way.

I hope to make more meaningful contributions to Comet but this is my humble start for now, keep up the great work!