Open the lock files close-on-exec

Each exec filter and snapshot compressor a fill runs inherited the slot's
lock descriptor beside the stdout it writes into. An flock lasts until
its last copy closes and nothing waits for a process a filter leaves
behind, so a fill killed before its rename could leave the slot locked,
and served stale or uncached, for as long as such a process lived.

The scan holds its lock across no exec and opens the same way so that
the two stay alike.

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