[arch-projects] [devtools] makechrootpkg: Keep everything inside of a function

[lukeshu at lukeshu.com]

I think that this is the last change from Parabola that would be
welcome in devtools.

It shoves the initial variable setup inside of a `init_variables()`
function, and the loose code at the end into a `main()` function,
which it calls as the very last line.

My motivation for this is that in Parabola, makechrootpkg is used as a
library, not as an executable, and so this makes adjusting it to be a
library as simple as removing the `main "$@"` line from the end.

However, I do think that the change stands on its own for 2 reasons;
using a `main()` function is good because
 - deferring execution until the entire script has been parsed avoids
   weird things happening if the user upgrades the program while an
   instance of it is running
 - generally good for code quality

Naturally, this re-indents a a decent size block of code, which means
that rebasing or merging with it is a pain.

-- 
Happy hacking,
~ Luke Shumaker