Properly handle child process exit codes

Properly handle child process exit codes

The child process 'exit' was returning the status of the process, rather than
the exit code. This patch properly deconstructs the status into the exit code
and the term signal a process may have received.

See:
http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod#Watcher_Specific_Functions_and_Data_-5
and waitpid(2)