When checking for updates in the background, close stdout. This prevents the
problem where the caller waits for the program 0launch ran to produce some
output, but the background thread is holding the pipe open.
This is only a partial solution. We should close all the other FDs too.
Also fixed the problem where any notification closing causes the background
process to exit, even if it wasn't the one with our 'Download' button.
git-svn-id: file:///home/talex/Backups/sf.net/Subversion/zero-install/trunk/0launch@1683 9f8c893c-44ee-0310-b757-c8ca8341c71e