adding timeout to webserver

after discovering a bunch of zombie webservers still running, I've added a
timeout option (inf for b/w compatibility), along with a sentinel file.
explicit termination via http://localhost:portnum/exit.m

Instead of relying on www/exit_server.m which kills matlab without the proper
cleanup, add a wart where http://localhost:port/exit.m is a 'speical' webpage
that signals webserver to shut down cleanly.