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Provide os/signal package that can be used to setup signal delivery to nogil
channels. This way for user code signal handling becomes regular handling of a
signalling channel instead of being something special or limited to only-main
python thread. The rationale for why we need it is explained below:

There are several problems with regular python's stdlib signal module:

1. Python2 does not call signal handler from under blocked lock.acquire.
   This means that if the main thread is blocked waiting on a semaphore,
   signal delivery will be delayed indefinitely, similarly to e.g. problem
   described in nexedi/nxdtest!14 (comment 147527)
   where raising KeyboardInterrupt is delayed after SIGINT for many,
   potentially unbounded, seconds until ~semaphore wait finishes.

   Note that Python3 does not have this problem wrt stdlib locks and
   semaphores, but read below for the next point.

2. all pygolang communication operations (channels send/recv, sync.Mut...