DBGSV_ERROR_OUT=shell already disables GUI message boxes on assertions,
but it does not take effect during early startup and late shutdown,
when the tools redirection hooks are not installed; setting
DISABLE_SAL_DBGBOX disables these on Windows in osl_assertFailedLine.

Change-Id: Ibb62054741e85dc966a36cb20515c3af8e0d9c11