Allow the directory for GUI executables to be specified as different
from console executables. The defaults for those two are different
on Mac OS, and configuring them differently might be useful to
address #1575.
Although there is probably no demand on Windows or Unix for splitting
the console and GUI bin directories, this patch tries to make things
work sensible there. On Windows, there's a corner case where a
launcher that starts GRacket (especially with `-z`) is intended to be
a console executable. The launcher creator can be told that via a
`subsystem` option, but a new `#:console?` argument was needed for
`make-gracket-launcher-path` lets the path selector know.
572b96a6ef