moved the temporary record buffer used in darshan-{convert,parser} into the heap; darshan_diff remains on the stack

this calculation was inadvertently accounting for metadata time
twice. also, clarified timing descriptions for slowest unique
file perf calculations.

mount information is now sorted in order of descending mount
point lengths to fix a bug in matching file names to mount
points. the longest mount point string which matches the file
name is the corresponding mount. this change requires slight
changes to the darshan-logutils API, but does not modify the
log file format

Otherwise, compile errors are generated when building an external
tool that uses the darshan-logutils api/library.