-Zhelp inconsistent with other unstable options

[Neved4]

Every other unstable option is separated by a space delimiter and -Z help fails,
only working properly when issued -Zhelp.

Related: #929

[pkgw]

Sorry for the long delay! This should have been fixed in #1084.

[Neved4]

Minor issue I noticed recently, but it might be good to tweak it for consistency's sake.

When running without a space delimiter:

$ tectonic -Zhelp
Available unstable options:
    -Z help                     Lists all unstable options

When running with a space delimiter:

$ tectonic -Z help

error: The following required arguments were not provided:
    <input>

When passing a foo extra argument it works:

$ tectonic -Z help foo

Available unstable options:
    -Z help                     Lists all unstable options

Also in tectonic --help:

-Z <option>...                      Unstable options. Pass -Zhelp to show a list

Should be -Z help.

Ultimately, the behaviour without space should be kept, otherwise it'd break backwards compatibility.