vchoi_fork

[svn-r12739] Added Direct I/O driver to VFD.  It's only supported by SGI Altix (cobalt).  There's a configure
option --enable-direct-vfd/--disable-direct-vfd to enable/disable Direct I/O support.  The default
is enabled.  There's a small test in test/vfd.c.  Another way to test it is to set environment
variable HDF5_DRIVER to "direct" and run "make check" in the test/ directory.  There'll be some
further improvement in the following checkin including allowing user to provide memory boundary
value, file block size, and copying buffer size.