[exim] notifier_socket and Linux namespace issues

Can anyone explain why Exim uses an abstract socket address on Linux for
notifier_socket?

It has some rather strange effects, because abstract sockets belong to
the network namespace, not the filesystem namespace.

The effect is that on most platforms, if you have multiple Exim
instances in different filesystem namespaces (e.g. chroot or containers)
then their notifier sockets are distinct; there is no crosstalk between
the instances.

However on Linux, if you have multiple Exim instances in different
filesystem namespaces, their notifier sockets collide, leading to the
'daemon_notifier_socket bind: Address already in use' error from all but
one of the Exim instances. There could also be crosstalk between the
instances, though I'm not sure what the socket is used for so I don't
know how much of a risk this is.

ttfn/rjk