[DO NOT MERGE} initial zsh support

- add zsh support to Terminal.vala
- start of zsh_startup.in script
  - basically copied from bash_startup.in
    - bashisms removed
    - use function autoloading instead of bash specific export -f

Still need to re-impliment Termlets in a zsh compatible manner, root out other bashisms that may still exist.

Also, still need to improve the multi-shell handling (not require using gsettings for setting the shell for example)