added support for DARSHAN_EXCLUDE_DIRS
Now users can set their own DARSHAN_EXCLUDE_DIRS. The syntax is comma separated list of directories: /dir1/,/dir2/,/dir3/...
Examples:
- to tell darshan not to exclude any system directory:
export DARSHAN_EXCLUDE_DIRS=none
- only exclude /etc/ and /proc/:
export DARSHAN_EXCLUDE_DIRS="/etc/,/proc/"