* siri/reltool/empty-radiobox/OTP-9384:
  Do not add an empty radio box on the releases tab for the start_clean release

First, the radiobox is changed to a listbox, since this will allow
multiple selections. This is however for future use - for now a
selection will only cause a printout in the erlang shell.

Second, add kernel and stdlib to the list of applications in order to
make the picture complete and avoid an empty list (radio) box for the
start_clean release.

If a module is duplicated in the library directories visible to
reltool, and the configuration does not point out which file to use,
then reltool:start will fail. This commit adds a pop-up which asks if
it should continue with a "safe" configuration:

 [{incl_cond,exclude},
  {app,kernel,[{incl_cond,include}]},
  {app,stdlib,[{incl_cond,include}]},
  {app,sasl,[{incl_cond,include}]}]

Earlier, reltool expected all module names detected under the lib
directories to have unique names. If this was not the case, the result
was undefined - i.e. the beam file of the duplicated module might be
included in multiple applications in the target area, or it might even
be excluded from all applications.

This commit adds awareness in reltool that a module might occur in
multiple applications, and it is allowed as long as the module or it's
application is explicitely excluded in all but one of the containing
applications.

This is the correction of the bug not allowing the values 'strip' or
'all' for the app_file option in reltool.

While at it, applied some cleanups and code modernizations suggested by tidier.