Updated preparation for dlsym offset env var.
Move FindHelperUtility to Util::getPath.
Use SharedData::installDir() to get dmtcp install dir.

This would allow us to use the current installDir upon restart instead
of using the hardwired one.
Fixed #18: Recalculate plugin paths on restart.
Fixed a rare segfault during process cleanup.

- See comments inside ckptThreadPerformExit() for more details.