dot/zshrc: Fix Emacs integration properly.
This was a mess. I'd had a hack to inhibit line-editing in `dumb'
terminals for ages, but grew an extra one as an overreaction against
TRAMP's allergy to bracketted-paste. Rationalize all of this.
* Move the logic to inhibit line-editing into the right configuration
section.
* Only inhibit line-editing in `dumb' terminals that aren't running in
Emacs.
* Inhibit bracketted-paste in `dumb' terminals, but not otherwise.
The result is that I can still use line editing in `dumb' terminals, but
Emacs -- and particularly TRAMP -- work properly.