run autoinstalls in the background, rather than aborting with timeout

[price]

Autoinstalls typically abort with a timeout, at varying stages of unfinishedness. I jacked up the FCGI timeouts a while ago; we may be running up against browser timeouts.

At a range of levels of ambition, we can

• show a static "we're working on it" page, autoinstall in the background, send mail when done
• show a "we're working on it" page that refreshes each N seconds to see if it's done; requires some mechanism for sharing that information between the remctl's thread and the refresh's thread
• show a "we're working on it" page that uses comet to wait for it to be done; requires the same info-sharing, plus comet.

[broder]

This is fixed by virtue of the autoinstaller being equivalent to an xm create operation.