Now that fsp's are created on successful file open, the structure member

Now that fsp's are created on successful file open, the structure member
fsp->open is no longer needed (if an fsp pointer is valid, then it's open :-).

NB for Luke, this patch also did not apply to TNG. TNG is not yet
identical w.r.t file serving with HEAD. This makes it impossible for
me to help maintain TNG. Please fix asap.

lib/substitute.c: Removed unused variable (pidstr).

Jeremy.