Add low-level access to fb_shutdown_callback API functions

[artyom-smirnov]

libfbclient handle signals on its own, so driver will get shutdown error on SIGTERM/SIGINT/SIGBREAK and there is no other possibilities to prevent it and softly stop process except handling shutdown manually:

Example of fb_shutdown_callback usage:

@FB_SHUTDOWN_CALLBACK
def driver_shutdown_callback(reason, mask, arg):
return 1 if reason == fb_shutrsn_signal else 0

def api_loaded_hook(api):
status = ISC_STATUS_ARRAY(20)
api.fb_shutdown_callback(status, driver_shutdown_callback, fb_shut_confirmation, None)
if db_api_error(status):
raise exception_from_status(InternalError, status, "Error while adding shutdown callback")

add_hook(APIHook.LOADED, FirebirdAPI, api_loaded_hook)

[artyom-smirnov]

Sorry, I accidentally closed previous PR by deleting branch. I've removed fb_shutdown in this PR (of course it already implemented in OO API, thanks) but kept fb_shut_* flags.