Add SA_RESETHAND (aka SA_ONESHOT) support.
Requested by: John Meacham

> module Main where

> import System.Posix.Signals

> main = do
>   installHandler sigUSR1 (Catch (print "usr1")) Nothing
>   installHandler sigUSR2 (CatchOnce (print "usr2")) Nothing
>   _ <- getLine
>   return ()