hdf5_msb

Implement H5EScreate, H5ESget_count, and H5ESclose.

It should be possible to write a simple application that creates an event
set and uses it with H5Fopen_async, H5Dread_async, H5Dwrite_async, and
H5Fclose_async, then calls H5ESclose (which waits for all async events to
complete).