(sigframe:;set): Call get_winapi_lock after frame is set so that signal handler
thread knows not to call SuspendThread.
(sigframe::~sigframe): Release winapi_lock.
* exceptions.cc (sigthread::get_winapi_lock): New method.
(sigthread::release_winapi_lock): New method.
(setup_handler): Use get_winapi_lock to ensure that signalled thread is not
blocked in a Windows API.
* path.h (path_types): Avoid broken GCC warning.