Fixes #281

From Francois Trahay

1) make sure wtime_offset is set properly when using MPI
2) fix ifdef logic to properly get wtime in all non-MPI cases

dynamic triggers still not working

- works for both mpi and non-mpi builds
- unlike darshan_core_time(), it does not compute relative timestamp and
  thus doesn't require darshan core to be initialized

- 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.

this allows us to request memory allocations at smaller granularity
than MiB

Signed-off-by: Cong Xu <cong.xu@intel.com>

Signed-off-by: Cong Xu <cong.xu@intel.com>

This is needed to distinguish between log files generated with
the same job id (i.e., for jobs that have multiple steps)

This function can be called by modules to allow FS-specific
modules to gather data for a given file path/file descriptor.
Currently integrated into POSIX and STDIO modules.