meek-client-torbrowser should exit on stdin close, even while waiting on browser output

Edit the browser extension not to output the meek-http-helper: listen line, or hack meek-client-torbrowser to break grepHelperAddress. Start Tor Launcher, select meek, and Connect. Now Cancel and exit Tor Browser. The bug is that meek-client-torbrowser and its child process firefox will continue running.

It happens because meek-client-torbrowser's TOR_PT_EXIT_ON_STDIN_CLOSE and SIGTERM logic happen only after grepHelperAddr. meek-client-torbrowser should pay attention to its stdin the whole time so that it can exit correctly in this case.