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.
% hg debuginstall
Checking encoding (ascii)...
Checking extensions...
Checking templates...
Checking patch...
Checking commit editor...
Checking username...
No problems detected
% hg debuginstall with no username
Checking encoding (ascii)...
Checking extensions...
Checking templates...
Checking patch...
Checking commit editor...
Checking username...
no username supplied (see "hg help config")
(specify a username in your .hgrc file)
1 problems detected, please check your install!