bring back default SIGINT handler

This seems to fix a problem I've recently seen where ctrl-c during rsync
leads to `git annex get` moving on to the next thing rather than exiting.

Seems likely that started happening with the switch to System.Process
(d1da9cf221), as the old code took care
to install a default SIGINT handler.

Note that since the bug was only occurring sometimes, I am not 100% sure
I've squashed it, although I seem to have.

Utility/SafeCommand.hs

@@ -49,8 +49,6 @@ boolSystemEnv command params environ = dispatch <$> safeSystemEnv command params
 safeSystem :: FilePath -> [CommandParam] -> IO ExitCode
 safeSystem command params = safeSystemEnv command params Nothing
 
-{- Unlike many implementations of system, SIGINT(ctrl-c) is allowed
- - to propigate and will terminate the program. -}
 safeSystemEnv :: FilePath -> [CommandParam] -> Maybe [(String, String)] -> IO ExitCode
 safeSystemEnv command params environ = do
 	(_, _, _, pid) <- createProcess (proc command $ toCommand params)