forkProcess fails with +RTS -N

I have a program in which I use `forkProcess`. When it is run with `-threaded +RTS -N1`, it works fine, but when I use `-threaded +RTS -N2` I get this error:

```
failed to create OS thread: Resource temporarily unavailable
```

I haven't been able to reproduce this issue in a small test case, only within the larger program. To provide some context though, it' called only from the following function:

``` haskell
pOpen :: FilePath -> [String] -> (Handle -> IO a) -> IO a
pOpen fp args func = do
pipepair <- createPipe
pid <- do
p <- Control.Exception.try (forkProcess $ childstuff pipepair)
case p of
Right x -> return x
Left (e :: Control.Exception.IOException) -> fail ("Error in fork: " ++ show e)
retval <- callfunc pipepair
let rv = seq retval retval
void $ getProcessStatus True False pid
return rv
where
callfunc pipepair = do
closeFd (fst pipepair)
h <- fdToHandle (snd pipepair)
x <- func h
hClose h
return $! x

childstuff pipepair = do
void $ dupTo (fst pipepair) stdInput
closeFd $ fst pipepair
closeFd $ snd pipepair
executeFile fp True args Nothing
```

GHC Version: 7.10.3
unix version: 2.7.1.0
OS: Ubuntu 16.04

Start at the pOpen entry point and its forkProcess call, then reproduce the larger program with GHC 7.10.3, unix 2.7.1.0, and +RTS -N1 versus -N2. Investigate the reported OS-thread creation failure and document a reproducible cause with a verified fix or limitation.

Written by the indexing model from the issue text.