Fixes #243

this requires a hacked up version of mdhim until there's a real api for
getting this information.

- reported by Jialin Liu
- triggered by some HDF5 programs
- solved by eliminating incomplete wrapper mechanisms for H5get_libversion()
  and invoking it directly (Darshan needs to use, not wrap, this
  function)

- match convention of other modules where possible
- avoid trampoline for non-PMPI calls
- resolve underlying symbols at run time
- use symbol name direclty in wrapper

Previously, modules would explicitly disable instrumentation when
ever Darshan core called into the module to shut it down. The
issue is that Darshan core starts shutting down well before modules
do, so there is a time interval where modules could be trying to
instrument data while its not safe to do so.

Now, we expose a function (darshan_core_disabled_instrumentation)
that modules can call to determine if its safe to instrument
data.