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.