`setSocketOption' fails with `Linger' option

[vvv]

setSocketOption uses FFI binding to setsockopt(2):

foreign import CALLCONV unsafe "setsockopt"
  c_setsockopt :: CInt -> CInt -> CInt -> Ptr CInt -> CInt -> IO CInt

This binding treats OPTVAL (4th argument) as a pointer to `int'. This is fine in most cases excepting the one when `SO_LINGER' option is being set. In the latter case OPTVAL must be a pointer to `struct linger'. (See libc manual.)

Haskell programs that try to set SO_LINGER socket option will successfully compile but fail in runtime:

$ runhaskell socket-linger.hs || echo X
$ runhaskell -DLINGER socket-linger.hs || echo X
socket-linger.hs: setSocketOption: invalid argument (Invalid argument)
X

$ gcc -g -Wall -W -o /tmp/1 socket-linger.c && /tmp/1 || echo X
$ gcc -g -DLINGER_AS_INT -Wall -W -o /tmp/1 socket-linger.c && /tmp/1 \
  || echo X
setsockopt: Invalid argument
X

(See the code of socket-linger.hs and socket-linger.c.)

Still, the necessity of `SO_LINGER' option is controversial. I think we better just remove `Linger' constructor from `Network/Socket.hsc'.