Linux abstract sockets are insecure as they lack any form of filesystem
permissions so their use allows anyone on the system to inject code into
the process.

This removes the default preference for abstract sockets in
multiprocessing introduced in Python 3.9+ via
https://github.com/python/cpython/pull/18866 while fixing
https://github.com/python/cpython/issues/84031.

Explicit use of an abstract socket by a user now generates a
RuntimeWarning.  If we choose to keep this warning, it should be
backported to the 3.7 and 3.8 branches.