searchidxshard: reuse $SIG{__WARN__} callback from Admin

We don't want to define $SIG{__WARN__} in the worker to call an
existing non-default callback.  Instead update ->{current_info}
the same way the V2Writable master process does.

I noticed this while reindexing with a large XAPIAN_FLUSH_THRESHOLD
and seeing the wrong epoch on my terminal from a shard because the
shard worker was spawned while reindexing a higher-numbered epoch.

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