event: allow to query userdata and watchdog state
event: add ability to change fd of an active event source
bus: rework how we attach fds to event loops

When trying a couple of addresses one after the other, make sure to
reattach the fds to the event loop.
bus: when a connection terminated use ECONNRESET as error
update TODO
build-sys: fix help text for --enable-kdbus
libsystemd-bus: the same error codes for sd_bus_release_name() (for kdbus and dbus1)

Due to this patch, sd_bus_release_name() function
returns the same code errors for kdbus and dbus1
if we try release non-existing name or foreign
name.
man: document more error codes for sd_bus_request_name()
nspawn: add new --setenv= switch to set an environment variable for the container to spawn
getty-generator: look add an environment variable $container_ttys set for PID 1 and start gettys on all ttys listed therein
man: document that systemd-getty-generator can start additional container gettys on ptys
update TODO
journal: downgrade comments about sd_journal_print_with_location()

Effectviely these calls are ABI anyway, so downgrade the comments a bit,
since it might actually be useful for language bindings to make use of
them.
event: make gcc shut up
getty-generator: fix segfault when $container_ttys is not set
bus: install systemd-bus-proxyd unit files for compatibility with dbus1
getty-generator: fix stripping /dev/
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