Debugger: Fix handling of team_id clashes.
TeamDebugger:
- Add accessor to query whether the debugger's interface is a post mortem core
rather than a live team.
TargetHostInterface:
- When asked to locate a team debugger by team_id, as is done for attach
requests to determine if we already have an existing instance attached, skip
over post mortems. This takes care of the potential problem of detecting a
core instance with the same team_id as a live team, and consequently refusing
the attach request.
- When a team debugger quits, use its actual pointer to look it up rather than its
team ID, as we could otherwise potentially remove the wrong one in the case of a
clash.