it's difficult for end users to find Darshan logs in system directory

Most sites document the Darshan log directory for their installation, but sometimes the path isn't easy to remember.

It would be nice if we had a simple way to report this path to the user. It could be an environment variable set by the software module, or maybe a utility could emit the compiled-in path. At any rate something that we could document in tutorials etc. that will work across sites without remembering the log path for each.

@robl and I noticed this as a user pain point at ATPESC 2018.