Various install-script fixes

* Colour handling in zsh (& don't use "colors" as funcname)

* Quote-protect -z test

* Clean up verbose var-printout

* git clone requires empty dir - tweak script to make it work with
  an existing dir

* Unquote asterisks for shell-expansion when byte-compiling
Merge pull request #568 from rowanthorpe/install-script-fixes

Various install-script fixes