propagate signals to the transferrer process group

Done on unix, could not implement it on windows quite.

The signal library gets part of the way needed for windows.
But I had to open https://github.com/pmlodawski/signal/issues/1 because
it lacks raiseSignal.

Also, I don't know what the equivilant of getProcessGroupIDOf is on
windows. And System.Process does not provide a way to send any signal to
a process group except for SIGINT.

This commit was sponsored by Boyd Stephen Smith Jr. on Patreon.

Types/TransferrerPool.hs

@@ -25,6 +25,10 @@ data Transferrer = Transferrer
 	{ transferrerRead :: Handle
 	, transferrerWrite :: Handle
 	, transferrerHandle :: ProcessHandle
+	, transferrerShutdown :: IO ()
+	-- ^ Closes the FDs and waits for the process to exit. 
+	-- Should be used when the transferrer is in between transfers,
+	-- as otherwise it may not shutdown promptly.
 	}
 
 newTransferrerPool :: IO TransferrerPool