A bunch of fixes etc. More stuff working, new stuff broken.

- Tracks a scan pointer of mail sub.
- Tracks \Seen flags for subs (not perm, based on scan pointer)
- Fix parsing of addresses in Envelope
- Use new (again) -1 value for subnum of mail
- Fix removal of whitespace at end of bodies
- Read all index entries by offset when base is selected rather than on demand
- Prevent base.open() from exiting the script on unknown sub