Split g96 I/O routines from confio.cpp

Split g96 I/O routines from confio.cpp

Move routines to read/write g96 files to g96io.*.
Ideally, these would be internal to the fileio/ module, but trjconv
has some ugliness.

Silence a pedantic clang warning that makes it impossible to minimize
header dependencies for mixed C/C++ code, and where problems would
anyways trigger as compilation failures in C code.

Change-Id: I20a88234bec8527f8fd41a614fb37e3144eb7532