zeroconf: suppress traceback during shutdown
If the read thred is in select when the main thread is in close, the main
thread may close the socket between select and read, generating a noisy
traceback. This can be ignored if the shutdown flag is set.
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<td class="description"><a href="/rev/1b0e7ece6bd6"><a href="http://bts.example.org/issue123">Issue123</a>: fixed the <i class="x">bug</i>!</a><span class="branchhead">default</span> <span class="tag">tip</span> </td>
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