Fix for issue 2616 (ldapsearch: error parsing command-line arguments)

Use a MultiChoiceArgument instead of a StringArgument for the scope argument.  The consequence of this is that the error message refers to the scope argument if no scope value is specified but the user provided the -s value.  In addition to this the code has been modified to allow specifying capital letters for scopes (so now -b ONE is accepted by the command-line).