emacs, zsh: Fix prompts for dumb terminals.

TRAMP seems to wet itself on my prompt---I can't think why---so use
the recommended default prompt from the TRAMP manual, modified for zsh
use.  Bash calls it "[\u@\h \w]$ ".  I call it a syntax error, like
the rest of bash.
zsh: Refactor display of 'loading' messages.
zsh: Detect and suppress loading messages on dumb terminal.
zsh: use an even dumber prompt.  Thanks, tramp.
emacs: Improve theme legibility