RunJava: handle directories in classpath properly

URLClassLoader determines whether URL denotes a jar or directory
by checking the last character of the path.

Before:

   [/]% java -cp java Hello
   Uncaught Java exception:
   java.lang.ClassNotFoundException: Hello
       at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
       at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
       at java.security.AccessController.doPrivileged(Native Method)
       at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
       at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
       at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
       at io.osv.RunJava.loadClass(RunJava.java:175)
       at io.osv.RunJava.runClass(RunJava.java:115)
       at io.osv.RunJava.parseArgs(RunJava.java:81)
       at io.osv.RunJava.main(RunJava.java:27)

After:

   [/]% java -cp java Hello
   Hello, world.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>