Only reset the signal to the default if we overrode them to begin.

When we don't override signal catching, we should presume the calling code
has installed custom signal catching hooks, and they won't want us to reset
the hooks after brokering has stopped.

Change-Id: I7fd80efdf3aa4bf04f5054361e862db2250a9b08
Merge pull request #48 from robertmaynard/server_easy_brokering_api

Only reset the signal to the default if we overrode them to begin.