Add support for the 'k' (kill) and 'D' (detach) packets (Jason Wessel).

Implement the 'k' gdbserial packet which kills the qemu instance via
the debugger stub.

Implement the 'D' detach packet for the gdb stub such that you can
disconnect gdb with the "detach" command.  This required implementing
a cpu_breakpoint_remove_all() and a cpu_watchpoint_remove_all()
function to cleanup all the breakpoints and watchpoints prior to
leaving the gdb stub else simulation can stop with no debugger
attached.

On a '?' packet remove all the breakpoints and watchpoints.  This is
considered more of a safety net in case you force killed gdb or it
crashed and you are reconnecting.  The identical behavior exists for
kgdb in the linux kernel.


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@4478 c046a42c-6fe2-441c-8c8c-71466251a162
