add alias for obscure 'v5r4' option name

[alanseiden]

IbmiToolkit/ToolkitApi/ToolkitServiceXML.php

Line 148 in 028e3a1

$opmString = ($this->getOption('v5r4')) ? " mode='opm'" : "";

The 'v5r4' option activates OPM mode, which is required for several features of XMLSERVICE. The name 'v5r4' appears obsolete to the uninitiated. I suggest we add an alias, i.e. 'opm', or something more clear, that does the same thing as 'v5r4'.