Simplify copying Java strings into memory allocated from a pool.

[in subversion/bindings/javahl]
* native/jniwrapper/jni_string.hpp (Java::String::strdup): New method.
* native/jniwrapper/jni_base.cpp (Java::String::strdup):
   Implement the method so that it automagically creates the string
   contents object and copies the contents to the pool.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1600851 13f79535-47bb-0310-9956-ffa450edef68