Refactor async mode to no longer use zpty

See technique used in `fast-syntax-highlighting`:
- ca2e18bbc9
- http://www.zsh.org/mla/users/2018/msg00424.html

Also see http://www.zsh.org/mla/users/2018/msg00432.html

In async response handler:
- We only want to read data in case of POLLIN or POLLHUP. Not POLLNVAL
  or select error.
- We always want to remove the handler, so it doesn't get called in an
  infinite loop when error is nval or err.

There is an upstream bug that prevents ctrl-c from resetting the prompt
immediately after a suggestion has been fetched asynchronously. A patch
has been submitted, but a workaround for now is to add `command true`
after the exec.

See https://github.com/zsh-users/zsh-autosuggestions/issues/364

src/start.zsh

@@ -14,10 +14,6 @@ _zsh_autosuggest_start() {
 	# zsh-syntax-highlighting widgets. This also allows modifications
 	# to the widget list variables to take effect on the next precmd.
 	add-zsh-hook precmd _zsh_autosuggest_bind_widgets
-
-	if [[ -n "${ZSH_AUTOSUGGEST_USE_ASYNC+x}" ]]; then
-		_zsh_autosuggest_async_start
-	fi
 }
 
 # Start the autosuggestion widgets on the next precmd