external dataset storage path behavior.
This check-in fixes a bug in the original check-in where the
external path stored in the file struct was not copied on reopen
causing subsequent dataset operations to fail.
Tested on: 64-bit Ubuntu 15.10 (Linux 4.2.0 x86_64) gcc 5.2.1
autotools serial w/ fortran and C++
autotools parallel (MPICH 3.1.4) w/ fortran