drop loopback out-of-process

[cgwalters]

This one may be a good first issue: today when doing install --via-loopback, if we get killed by e.g. Ctrl-C (i.e. SIGINT) we will leak the loopback device allocation.

I've been thinking here what would be nice in general is to create a little "out of process drop" helper that could handle these external resources; it would:

• mask most signals that would otherwise be fatal
• own the resource and if the parent process died (via PR_SET_PDEATHSIG) we'd take care of dropping them

This type of flow would also be nice for temporary directories, which are also easy to leak in this way.

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Hmm, we may also be able to stop forking /bin/losetup and instead use the ioctl APIs and only hold a file descriptor, but that's a bigger change.