NarProcessLibraries enhanced command construction

Updated the nar-process-libraries mojo to allow more flexibility when
constructing the command line to be executed.
As a side effect, the configuration required has been updated and documented
accordingly.  An example is given below:

<processLibraries>
    <token/>
    <commands>
        <command>
            <libraryType/>
            <executable/>
            <arguments>
                <argument/>
            </arguments>
        </command>
    </commands>
</processLibraries>