use vfork instead of fork on QNX

On QNX, fork cannot be used in multithreaded programs, but vfork can,
so that's what we'll use.

http://www.qnx.com/developers/docs/6.4.1/neutrino/getting_started/s1_procs.html

classpath/java-lang.cpp

@@ -402,7 +402,16 @@ Java_java_lang_Runtime_exec(JNIEnv* e, jclass,
     return;
   }
   
+#ifdef __QNX__
+  // fork(2) doesn't work in multithreaded QNX programs.  See
+  // http://www.qnx.com/developers/docs/6.4.1/neutrino/getting_started/s1_procs.html
+  pid_t pid = vfork();
+#else
+  // We might be able to just use vfork on all UNIX-style systems, but
+  // the manual makes it sound dangerous due to the shared
+  // parent/child address space, so we use fork if we can.
   pid_t pid = fork();
+#endif
   switch(pid){
   case -1: // error
     throwNewErrno(e, "java/io/IOException");