crypto: cleanup NSPR in main thread

crypto: cleanup NSPR in main thread

quote from nspr's header file

```
 * Perform a graceful shutdown of NSPR.  PR_Cleanup() may be called by
 * the primordial thread near the end of the main() function.
```

this helps to silence some warnings from valgrind. but it does not hurt
in practice, because the process is about to die. and the freed memory
chunks are only allocated once in NSPR.

Signed-off-by: Kefu Chai <kchai@redhat.com>