New option `haskell-process-log` for controlling "*haskell-process-log*"

The primary purpose of "*haskell-process-log*" is for debugging
communication with the inferior GHCi process, however the buffer keeps
filling up without bounds and is of little value for
non-developers. Thus, the default setting of the new
`haskell-process-log` is `nil` which disables logging to
"*haskell-process-log*".