tool: call PR_Cleanup() on exit if NSPR is used

This prevents valgrind from reporting possibly lost memory that NSPR
uses for file descriptor cache and other globally allocated internal
data structures.

RELEASE-NOTES

@@ -36,6 +36,7 @@ This release includes the following bugfixes:
  o Curl_rand: Use a fake entropy for debug builds when CURL_ENTROPY set
  o nss: do not abort on connection failure (failing tests 305 and 404)
  o nss: make the fallback to SSLv3 work again
+ o tool: prevent valgrind from reporting possibly lost memory (nss only)
  o 
 
 This release includes the following known bugs:

src/tool_main.c

@@ -27,6 +27,10 @@
 #include <signal.h>
 #endif
 
+#ifdef USE_NSS
+#include <nspr.h>
+#endif
+
 #define ENABLE_CURLX_PRINTF
 /* use our own printf() functions */
 #include "curlx.h"
@@ -205,6 +209,11 @@ static void main_free(struct GlobalConfig *config)
   curl_global_cleanup();
   convert_cleanup();
   metalink_cleanup();
+#ifdef USE_NSS
+  if(PR_Initialized())
+    /* prevent valgrind from reporting possibly lost memory (fd cache, ...) */
+    PR_Cleanup();
+#endif
   free_config_fields(config);
 
   /* Free the config structures */