slightly improve finding the py4j jar file; allow PY4J_JAR environ var
slightly improve finding the py4j jar file; allow PY4J_JAR environ var

MSLNZ/msl-loadlib,MSLNZ/msl-loadlib,MSLNZ/msl-loadlib,MSLNZ/msl-loadlib,MSLNZ/msl-loadlib