The command line parameter -userid=.. does not work as advertised in the
document at <URL (see above)>. There are two ways to fix this: One is to correct
that document, the other is to reactivate the functionality. I propose to do
that latter and actually can propose how a simple fix should look like.

From a glance at the code: This parameter is still parsed, but it is then passed
to code that does not use it, except in some webtop-specific code that is not
even built.

Instead the info where the user-installation is, is taken from the bootstrap
parameter 'UserInstallation' which is read from $SYSBINDIR/bootstraprc (or
bootstrap.ini on non-unx systems) in the normal case.

There already is a way to override this on the command line by specifying
-env:UserInstallation=<file-url>. But this method is tedious, because the user
needs to specify a correct file URL (as required by osl). Thus it makes sense to
revive the -userid= parameter, if it can be made to accept ordinary
system-specific pathes.

This can be done as follows:

  if (GetCommandLineArgs()->getUserDir(aUserDir))
  {
    OUString aUserDirURL;
    if (osl::File::getFileUrlFromSystemPath(aUserDir,aUserDirURL) != 0)
         /* Handle the error */;

    // now must be a valid file URL. For best results make absolute using
    //
osl::File::getAbsoluteFileURL(getProcessWorkingDirectory(),aUserDirURL,aUserDirURL)
    
    // now override the bootstrap setting:
    rtl::Bootstrap::set( OUString("UserInstallation"), aUserDirURL)
  }

This change of bootstrap parameters should of course be done before trying to
use the setting. This means the change should at least be done before
initializing UNO.