user modules can be used only in othervm

usage of user modules leads to CNF if a test is run in agentvm, because jtreg doesn't communicate module path to agents.

A similar problem exists in driver actions, even in othervm execution mode, as DriverAction filters out --module-path and other related flags (rerun section, though, has them listed which made it hard to comprehend)

http://cr.openjdk.java.net/~iignatyev//jtreg/modules-in-agent/webrev.00.test contains tests that illustrate the issue. e.g. mainAction/usermods/RunUserMod, when run w/ -agentvm, fails due to:
 
java.lang.ClassNotFoundException: um1_p1.um1_p1_M1
        at java.base/java.net.URLClassLoader.findClass(URLClassLoader.java:466)
        at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:563)
        at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:496)
        at com.sun.javatest.regtest.agent.MainActionHelper.runClass(MainActionHelper.java:143)
        at com.sun.javatest.regtest.agent.AgentServer.doMain(AgentServer.java:301)
        at com.sun.javatest.regtest.agent.AgentServer.run(AgentServer.java:232)
        at com.sun.javatest.regtest.agent.AgentServer.main(AgentServer.java:69)

JavaTest Message: main() method must be in a public class named
JavaTest Message: um1_p1.um1_p1_M1 in file um1_p1.um1_p1_M1.java