added a @NoJPFExecution annotation, which sets a NoJPFExec system attr on marked methods during class load time, which gets checked by ThreadInfo.enter(). Useful to flag methods which have to be intercepted/cut off when executing classes under JPF that can also be used outside. Especially useful to avoid the recursive JPF problem that can be caused by tests (which mix classpath and native_classpath). This currently throws a JPFException, but we could also turn this into a AssertionError in the SUT so that we get the SUT stack trace