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 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.

rather than using 'begin_shutdown', 'get_output_data', and
'shutdown', we combine all three into one shutdown routine.

Cleanup the structure and documenation of the code to be in sync
with the new mmap version of Darshan. In particular, I made sure
all extra mmap logic was properly ifdef wrapped so it's more clear
which portions of the code are responsible for implementing that
feature.

This interface provides some convenience functions for mapping
opaque record identifiers (could be a filename, file descriptor,
or any other type of file handle) to a corresponding runtime
structure referencing the actual file record. This runtime
structure should at least contain a pointer to the actual file
record, and can also persist arbitrary state for this record.

- memory management is now entirely handled by darshan-core for
  module records
    * when registering new records, darshan-core returns the
      corresponding buffer to write it to, rather than allowing
      module to determine this
- modules now decide whether darshan-core should track record
  names by passing either a valid name or NULL to the register
  function
    * this allows modules to register records that aren't really
      tracked by darshan-core
- only the POSIX module has been ported to new changes for this
  commit

the POSIX module will call into the Lustre module for each file
record created that corresponds to a file from a Lustre FS. No
counters are currently stored but most of the module strucutre
has been stubbed.

- modules now receive a memory buffer after successfully
registering with darshan-core. modules can request a specific
memory amount, but may not be honored depending on the configured
darshan memory limit
- split the previous register_record function into 2 parts:
    * lookup() -> lookup darshan_record_id corresp. to path name
    * register() -> register record_id & name with darshan-core
- register_record now returns true if the record added; false
otherwise (if module was out of memory or darshan is out of memory
for storing id->name mappings)

Now, register_record() accepts an argument for the size of
the record being registered, and this is used to track how much
data each module has stored in their allocated memory region.

user can now give a config option or use a runtime environment
variable to control darshan's per module memory requirements.