The symbols HAVE_SIGACTION and BUSFAULT are set under the same conditions,
so can be consolidated into one. Also define dummies when HAVE_SIGACTION
is not set, so a few #ifdef's less clutterig the code.
Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1297>