Improve install script a bit

* Set ENVPATH with the help of readlink and OS-specific switches
* Add PIP var since RHEL/EL* has to be special and needlessly
  calls the command `pip-python`
* Back out of _install_syspackages if the OS .packages-* file
  doesn't exist
* Add ~/.zsh directory symlink in _install_dotfiles
* Move chsh zsh stuff to a function to clean up the main portion
  of the script a bit