`install-tl --help` doesn't work anymore because of gui

[jack]

There is a bug that I think results from the fact that using the gui of
`install-tl` is now the default on macOS.

That bug is the following: the user now can't just do `./install-tl
--help` to get a list of available options.  If I try that, a gui gets
launched (which, to add insult to injury, doesn't even show a list of
options).  Even if I add to that command the string "-gui=text" it still
doesn't work.

Please bring back the ability to do a plain `./install-tl --help` and see,
*in the terminal*, a list of available options.

(Surely, for instance, the user should be able to do that in order to find
out what flag will turn *off* the gui!)

-jack