Based on https://github.com/Valodim/zsh-capture-completion

`zpty -r` with a pattern seems to have some funky behavior on older
versions, giving unpredictable results

Don't use `-s` option to `zmodload`. It is not available in zsh versions
older than 5.3

If running in sync mode and a completion takes a long time, the user can
^C out of it. We need to use `always` in the strategy function or the
pty will not be destroyed in this case and the next time we go to create
it, it will fail, making the shell unusable.

User can have many different completion styles set that will modify what
they've already typed. These styles will result in suggestions that
don't match what the user has already typed. We try our best to unset
some of the more problematic ones, but add some code to fetch to
invalidate suggestions that don't match what the user's already typed.

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

They're not quite ready. Keep them on a feature branch for now.

Not needed after move away from zpty for async

This reverts commit 7f8ff2867c.

Maybe this is also a fix for #247, #248 and #258. Supersedes #267.

Testcase:
Using match_prev_cmd strategy and with these lines in history:
echo '1^'
echo '2^'
echo '1^'

type:
echo       (unexpected suggestion echo '1^' instead of echo '2^')
echo '1^1  (wrong suggestion echo '1^1echo '1^')
echo '1^#  (error "bad math expression")

These options have been deprecated for over a year.

Checks whether `zpty` gives a file descriptor, which was not the case in
older versions of zsh.

Based on a4b2f81c96/async.zsh (L395-L401)