New option HASKELL-INTERACTIVE-MODE-INCLUDE-FILE-NAME

Personally, I intentionally excluded the file-name from
HASKELL-INTERACTIVE-SHOW-LOAD-MESSAGE, because it's 99% of the time
useless information. @hvr added it in there, so I've made it an
option, enabled by default.
FOO -> `foo` convention tweak.
Re-use the same window for `haskell-present` calls from a presentation buffer.

Not sure whether this is good or not. Seeing how it feels.
Check whether PROCESS is nil in `haskell-process-set'.

Sometimes if a session is a little messed up, a nil process can make
it difficult to kill the session due to the `setcdr' calls.