POSIX_F_OPEN_TIMESTAMP is now POSIX_F_OPEN_START_TIMESTAMP
POSIX_F_CLOSE_TIMESTAMP is now POSIX_F_CLOSE_END_TIMESTAMP

I also added POSIX_F_OPEN_END_TIMESTAMP and
POSIX_F_CLOSE_START_TIMESTAMP, so we can better understand how
long open and close phases are for some applications.

We don't currently have a system to try this out on, but just
wanted to make sure something was there as a failsafe. The current
implementation should be able to correctly aggregate all counters
in BGQ records, except for BGQ_INODES.

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.