proxy.config.stack_dump_enabled

While debugging the issues we've had with ATS, we needed to generate core files, but by default ATS handles the SIGSEGV signal and prints the backtrace to traffic.out. We've find out, though, that there's a config setting that disables this behavior, allowing core files to be generated, and decided we should add this information to the doc as others, specially newcomers to ATS, may find it useful.