makechrootpkg: Avoid having code floating around outside of a function.

This means wrapping variable initialization in init_variables(), and the
main program routine in main().

I did NOT put `shopt -s nullglob` in to a function.

It make make sense to move init_variables() down into the main()
function, instead of having it as a separate function up top (if this
done, then the `-g` flag passed to `declare` in init_variables() can
be dropped).  However, in interest of keeping the `diff -w` small, and
merges/rebases simpler, this isn't done here.