Another RRDP review, startup

Another RRDP review, startup

- Improve usage of `xmlChar *`
  (Was being casted to/from `char *` rather contract-breakingly.)
- Bunch of renames
	- notification_metadata -> rrdp_session
	  (These are not exclusive to Notifications.)
	- delta_head -> notification_delta
	  (The object specifically refers to Notification delta tags,
	  I don't know what "head" is supposed to allude.)
	- rdr_snapshot_ctx, rdr_delta_ctx -> rrdp_ctx
	  (Slightly tweaked the semantics of this, to reduce argument
	  lists.)
- Remove redundant fnstack pushes and pops
  (Rather comically, snapshots and deltas were being stacked twice:
  parse_snapshot() + rrdp_parse_snapshot(), parse_delta() +
  process_delta().)

There are still a few FIXMEs in RRDP yet.