Add Packfile Segment in PIR

[Mark Glines]

Mark Glines wrote:
> The attached patch, applied on top of yours, gets the test passing.

Hmm.  On further reflection, it might be better to put those changes to
the init vtable straight into Segment, because creating PF_DIR_SEG
segments by default isn't very useful.

At the end of the day, every segment subclass will need to set its own
type up properly, which is why I initially did it like that.  But
Segment should probably default to PF_UNKNOWN_SEG, which would mean
RawSegment doesn't need to do anything.

I haven't really thought about the details of making these structures
writable before, for the first stage I've been concentrating entirely on
making existing structures readable.  Thanks for stirring the pot.

Mark