bus: when connecting to a container's kdbus instance, enter namespace first

Previously we'd open the connection in the originating namespace, which
meant most peers of the bus would not be able to make sense of the
PID/UID/... identity of us since we didn't exist in the namespace they
run in. However they require this identity for privilege decisions,
hence disallowing access to anything from the host.

Instead, when connecting to a container, create a temporary subprocess,
make it join the container's namespace and then connect from there to
the kdbus instance. This is similar to how we do it for socket
conections already.

THis also unifies the namespacing code used by machinectl and the bus
APIs.
util: when joining a namespace make sure to reset all uids to 0 after
the transition
util: port last code over to new namespace utility calls
event: dynamically adjust size of events array instead of pre-allocating it possibly too large or too small
event: instead of reset the revents field when we get new revents data from epoll, OR it in
event: some snake-oil to speed up impossible error checks