commandserver: extract handler of new socket connection This prepares for adding an IPC socket.

commandserver: pass around option to hook repo instance creation This is necessary to wrap a repo instance so the master process will be notified on repo.close().

py3: stop subscripting socket.error In 3.3 and later, this is now an alias for OSError. I hacked up the server code enough that I was able to trigger the exception handler in server.py from test-http-bundle1.t. Other instances of this either subscript through the `args` member, or reference the errno or strerror attributes. Note that on Windows, the errno value seems to reflect the Winsock error, so the various tests for EPIPE seem like they would always fail. But that seems to be the case in py2 as well.