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
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