this includes defining an example logutils function for
aggregating null records into a single shared null record (needed
for darshan-merge utility)

also, bug fix to avoid any attempt of reading log file for
modules with no data present in the log

gkl/obfuscate fs fix: prepend file path to hashed file name to retain fs info

Prepends the file system path to the hashed file name generated by `darshan-convert --obfuscate` so that darshan-parser can still link obfuscated file records to the mounts table.

See merge request !1

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.

- remaining items are tracked in gitlab issues

- untested