commandserver: unblock SIGCHLD This enables the SIGCHLD handler to work properly if some buggy program started chg server with SIGCHLD blocked. A test of this probably requires C code, but we don't have such kind of tests already. Since this is a simple and clear fix, I'm leaving it as "untested" but I did a manual test and there were no longer zombie workers. Differential Revision: https://phab.mercurial-scm.org/D1737