in some cases abuf can be allocated earlier to avoid a memcpy

git-svn-id: https://svn.mcs.anl.gov/repos/parallel-netcdf/trunk@1963 c368ceb8-fd28-0410-a457-e46ee1d7cd4a
add a new test program flexible_bput.c

git-svn-id: https://svn.mcs.anl.gov/repos/parallel-netcdf/trunk@1964 c368ceb8-fd28-0410-a457-e46ee1d7cd4a