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- read and write start timestamps should record start timestamp of first
  read/write to *begin*, not start timestamp of first read/write to
  complete
- mirrors earlier fix to open timestamps

- be careful to record start timestamp of first open to start rather
  than start timestamp of first open to complete

- move fopen/fopen64 wrappers to stdio module
- make infrastructure for tracking stream pointers based on similar fd
  infrastructure in posix
- define placeholder record structs
- wrappers don't actually do anything yet

Darshan now provides debugging tips for cases where log files
are not generated.

Since we had to modify HDF5/PnetCDF linker flags to work on Cray
by undefining appropriate symbols, we need to update the linker
options used in darshan-config. darshan-config linker options need
to include the stubs libraries and use '--start-group' and
'--end-group' flags to resolve the symbols correctly

this reorganization prevents darshan from being "enabled"
(i.e., accessible to modules) until the initialization
has completely finished.

- prevents inadverdent disabling of Darshan on some MPICH builds
- fixes #94