of failing to check negative-wraparound of int32 operands.
- Add casts and XXX comments to MSG_* calls that lack prototypes.
- #ifdef XP_UNIX around MimeHeaders_do_unix_display_hook_hack.
Mostly I did this by turning on -DMOZILLA_30, which eliminates most of
the, uh, "progress" made in libmime since 3.0. That this worked highlights
the fact that the 3.x libmime had far fewer cross-dependencies on libmsg
than 4.x did.
