Posix.runProcess

[George Russell]

I think Posix.runProcess should give access to the eventual exit state of
the process, if any.  Two possible ways:

(1) Change its result to IO (IO ExitCode), the inner action being something
    that waits for the process to complete (without blocking all other threads!)
    and returns the ExitCode.  
    (Even better would be to make the result just IO ExitCode but this might
    be too incompatible for people who rely on runProcess returning at once.)
(2) If (1) can't be done, change its type to return something similar to
    Posix.getProcessStatus.