Suggested fix for JDK-4724038 (Add unmap method to MappedByteBuffer)

[Dawid Weiss]

> there're already ways to bring down the JVM (e.g. JNI, existing Unsafe usage, etc),

If you can somehow pass switches to the JVM, this one wins my personal
beauty contest (from globals.hpp):

product(intx, SelfDestructTimer, 0,                                       \
          "Will cause VM to terminate after a given time (in minutes) "     \
          "(0 means off)")

Dawid