Check return code of uloop_run() and pass the signal number up when the
loop was interrupted. After cleanup, uninstall uloop's signal handlers
and re-raise the signal to terminate the process.
This allows interrupting the autoupdater using Ctrl-C during downloads,
instead of having it continue with the next mirror (if multple are
configured). As uloop's signal handlers only set a flag to interrupt
uloop_run() and have otherwise no effect, the autoupdater can still only
be interrupted during HTTP requests, ensuring we can't leave the system
in an inconsistent state.
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