Add system SQL/J jars, just like system routines. 

Pretty straightforward copy/paste of parallel code. The justification is to allow plugins to define system stored procedures, specifically Java stored procedures that call back to the same classes as the plugin itself. Since these classes are not in the system class path, SQL/J jar handling is needed for the rendezvous.

Since work on plugins is on-again / off-again, we really should have a test plugin as part of must-pass that just checks things like service startup, and now, this, for regressions. In the meantime, this new code does work for me.