A target has-a 1:N threads?

[Andrew Cagney <ac131313@redhat.com>]

Hello,

At present GDB maintains a thread-db that contains all the "threads" 
known to GDB.  The list is cross-target.  That is, it can include both 
LWPs from the lower "process layer" and "threads" from the higher 
"thread layer".

I think the currently implicit target <-> thread relationship should be 
made more explicit:

- thread_info should point back at it's target
- a target should let you iterate over its "threads"

This, I belive, will help clean-up the current tid:pid dance.  A thread 
would always know its target and hence thread operations would always be 
applied at the correct level.

thoughts?
Andrew