Add a --silent mode to add-user.[.sh, .bat] script

Add a --silent mode to add-user script. This mode is only valid in non-interactive mode, i.e if at least a username and password are provided as arguments to that script. SO running:

./add-user.sh --silent=true

will still allow the script to print. [silent mode is disabled in that case]

However running:

./add-user.sh --silent=true <username> <password> [ManagementRealm]

will make the script silent.
NOTE that it won't even print error messages if --silent=true is put.

The – option can be placed anywhere.
So this will also work:

./adduser.sh <username> --silent=true <password>

or

./adduser.sh <username> <password> <ManagementRealm> --silent=true

The silent option can be useful in scripts that call the add-user script. The silent mode disables the check for a console object since we don't need to print anything to the console.

We need the silent option since the way IzPack [GUI Installer] calls scripts, the Console object is not available and the add-user script will fail.