new breakpoint type for thread events

[Michael Snyder <msnyder at cygnus dot com>]

All, 

I have added a new breakpoint type in enum bptypes:
bp_thread_event.  This breakpoint will be an internal
breakpoint, invisible to the user, which the target
implementation can set in order to get notification
of events such as new thread creation.  Some thread
libraries handle debugger notification by giving the
debugger a magic location at which to set a breakpoint.
This location will be "called" whenever there is 
something interesting for the debugger to notice
(such as a new thread).

In breakpoint.c I've added create_thread_event_breakpoint,
remove_thread_event_breakpoints, and appropriate switch
cases and actions to make these breakpoints invisible to
the user.  When the event loop detects this breakpoint,
it should just continue over it without notifying the
user or taking any action; it is up to the target layer
to detect this "special" breakpoint and take whatever
actions are appropriate (such as scanning for new threads
and adding them to GDB's thread list).

This should show up on the external CVS server
sometime soon.

				Michael Snyder