`opts_pre` was re-declared empty between first definition and use in a
second check, so was a no-op in that second check.

The parsing of `opts_post` would stop at the first "non-help" option,
meaning a `--help` or `-h` specified further down the command-line would
not be seen.

Before:

```
russell@zip:~ ❯ znap pull --foo --help
russell@zip:~ ❯ znap pull --help --foo
Usage: znap pull [ <repo> ... ]
Update repos in parallel.
russell@zip:~ ❯ znap pull bla bla --help --foo
russell@zip:~ ❯ znap pull bla bla --foo
russell@zip:~ ❯ znap pull
znap no upstream
znap-shallow ahead 3
```

After:

```
russell@zip:~/src/zsh-snap ❯ znap pull --foo --help
Usage: znap pull [ <repo> ... ]
Update repos in parallel.
russell@zip:~/src/zsh-snap ❯ znap pull --help --foo
Usage: znap pull [ <repo> ... ]
Update repos in parallel.
russell@zip:~/src/zsh-snap ❯ znap pull bla bla --help --foo
Usage: znap pull [ <repo> ... ]
Update repos in parallel.
russell@zip:~/src/zsh-snap ❯ znap pull bla bla --foo
russell@zip:~/src/zsh-snap ❯ znap pull
znap no upstream
znap-shallow ahead 3
```

Fixes marlonrichert#205

Test:

```russell@zip:~ ❯ cat test.zsh
t() {
    path=(/usr/bin /bin)
    fpath=()
    echo --
    echo znap install "$@"
    znap install "$@"
    echo path = $#path , fpath = $#fpath
}

t asdf-vm/asdf --no-path --no-fpath
t --no-path --no-fpath asdf-vm/asdf
t --no-path asdf-vm/asdf
t asdf-vm/asdf --no-path
t --no-fpath asdf-vm/asdf
t asdf-vm/asdf --no-fpath
t asdf-vm/asdf
t asdf-vm/asdf --no-such-arg
russell@zip:~ ❯ source test.zsh
--
znap install asdf-vm/asdf --no-path --no-fpath
path = 2 , fpath = 0
--
znap install --no-path --no-fpath asdf-vm/asdf
path = 2 , fpath = 0
--
znap install --no-path asdf-vm/asdf
path = 2 , fpath = 1
--
znap install asdf-vm/asdf --no-path
path = 2 , fpath = 1
--
znap install --no-fpath asdf-vm/asdf
path = 3 , fpath = 0
--
znap install asdf-vm/asdf --no-fpath
path = 3 , fpath = 0
--
znap install asdf-vm/asdf
path = 3 , fpath = 1
--
znap install asdf-vm/asdf --no-such-arg
znap clone: invalid argument '--no-such-arg'
Usage: znap clone ( <user>/<repo> | <url> ) ...
Download repos in parallel.
path = 2 , fpath = 0
```