zsh: backup and set shell options that affect PS1 evaluation
zsh: backup and set shell options that affect PS1 evaluation

We use zsh default options (promptpercent, nopromptbang, nopromptsubst)
anyway, but just in case, backup what existed before our loading and
restore it in prompt_off/prompt_OFF.

dtschan/liquidprompt,smadhueagle/liquidprompt,joshuarubin/dotfiles,nojhan/liquidprompt,dpilone/liquidprompt,pkkolos/liquidprompt,laurentbardin/liquidprompt,dan-cleinmark/liquidprompt,jaesivsm/liquidprompt,milouse/liquidprompt,joshuarubin/dotfiles,tsbarnes/liquidprompt,joshuarubin/dotfiles,BobHood/liquidprompt,joshuarubin/dotfiles,alexprengere/liquidprompt