We are silently ignoring `UserInterrupt`s on `delegate_ctlc`

[sol]

When delegate_ctlc is used then waitForProcess throws UserInterrupt if the child terminated due to ctrl-c.

However, we ignore any such exceptions.

1. We call waitForProcess here

   typed-process/src/System/Process/Typed.hs

   Line 226 in 788209f

   then P.waitForProcess pHandle

2. Any exceptions will then be turned into a Left-value here

   typed-process/src/System/Process/Typed.hs

   Line 250 in 788209f

   eec <- waitCatch waitingThread

3. Finally we ignore them here

   typed-process/src/System/Process/Typed.hs

   Line 258 in 788209f

   Left _ -> do

   (wrongly assuming that it can only be an AsyncCancelled)

#70 addresses this specific problem.

However, we also deadlock on waitExitCode with delegate_ctlc on ctrl-c, as in that case pExitCode will be left empty:

$ cabal exec -- ghci -isrc src/System/Process/Typed.hs
ghci> withProcessWait (setDelegateCtlc True $ proc "sleep" ["100"]) $ \ p -> waitExitCode p
^C
*** Exception: thread blocked indefinitely in an STM transaction

(I'm using cabal exec instead of stack exec here, as stack exec fails with a different error. I assume that this is due to stack using typed-process itself and hence it is itself affected by this bug and #69.)

You could address this by say storing an Either SomeException ExitCode in pExitCode, but async already solved this for us. So I think we should rely on async instead.

Once we are done with #70, I'll open a PR along the lines of sol@3e37a48, which addresses this issue.