worker: make sure killworkers() never be interrupted by another SIGCHLD
killworkers() iterates over pids, which can be updated by SIGCHLD handler.
So we should either copy pids or prevent killworkers() from being interrupted
by SIGCHLD. I chose the latter as it is simpler and can make pids handling
more consistent.
This fixes a possible "set changed size during iteration" error at
killworkers() before cleanup().
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).