This makes sure that system APC, such as APC_BREAK_PROCESS do not get
interrupted in the middle of their execution, and that the APC
completion notification is always correctly sent back to the caller.

Otherwise DbgBreakProcess sometimes did not return until
WaitForDebugEvent/ContinueDebugEvent are called, because of a race
condition between the APC servicing thread, and the newly created
exception thread.
Signed-off-by: Rémi Bernon <rbernon@codeweavers.com>
Signed-off-by: Jacek Caban <jacek@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>