Enable parallel access for pnetcdf mode.

configure

configure.ac

src/cdf_int.c

src/cdf_int.h

src/cdf_write.c

src/config.h.in

src/namespace.c

src/namespace.h

src/pio_cdf_int.c

src/pio_server.c

src/stream_cdf_o.c

src/stream_cdf_time.c