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

spec/options/async_zpty_name_spec.rb

@@ -1,19 +0,0 @@
-context 'when async suggestions are enabled' do
-  let(:options) { ["ZSH_AUTOSUGGEST_USE_ASYNC="] }
-
-  describe 'the zpty for async suggestions' do
-    it 'is created with the default name' do
-      session.run_command('zpty -t zsh_autosuggest_pty &>/dev/null; echo $?')
-      wait_for { session.content }.to end_with("\n0")
-    end
-
-    context 'when ZSH_AUTOSUGGEST_ASYNC_PTY_NAME is set' do
-      let(:options) { super() + ['ZSH_AUTOSUGGEST_ASYNC_PTY_NAME=foo_pty'] }
-
-      it 'is created with the specified name' do
-        session.run_command('zpty -t foo_pty &>/dev/null; echo $?')
-        wait_for { session.content }.to end_with("\n0")
-      end
-    end
-  end
-end