commandserver: extract _cleanup() hook to clarify chg is doing differently This makes it clear that chg needs its own way to unlink closed socket file. I made a mistake in draft patches without noting the difference.

chgserver: drop repo at chgunixservice.__init__() Since it isn't expensive operation, we don't have to delay it to init().

chgserver: extract utility to bind unix domain socket to long path This is common problem of using sockaddr_un.

chgserver: narrow scope of chdir() to socket.bind() This helps extracting a utility function.