"exclusive = True" may lead to undefined behavior #58

It's not very well documented, because O_CREAT is not passed like the other flags via OpenFileFlags, but implicitly via Maybe FileMode.

The manpage says that

If O_EXCL is set and O_CREAT is not set, the result is undefined.

This library should either make the correspondence of those input arguments clear or make it impossible to use the combination. The following would already be quite an improvement:

-- |Correspond to some of the int flags from C's fcntl.h.
data OpenFileFlags =
 OpenFileFlags {
    append    :: Bool,           -- ^ O_APPEND
    exclusive :: Bool,           -- ^ O_EXCL, result is undefined if O_CREAT is False
    noctty    :: Bool,           -- ^ O_NOCTTY
    nonBlock  :: Bool,           -- ^ O_NONBLOCK
    trunc     :: Bool,           -- ^ O_TRUNC
    creat     :: Maybe FileMode  -- ^ O_CREAT 
 }

And then remove the extraneous Maybe FileMode arguments from open_.