networkd: avoid segfault
networkd: make sure Network and Link can always be freed

Also, don't fail a link just because we can't find its MAC address.
bus: the :no-sender hack is now unnecessary, since the new library doesn't require this anymore
manager: always sort environment block, it's prettier
hostnamed: modernizations
bus: rework message handlers to always take an error argument

Message handler callbacks can be simplified drastically if the
dispatcher automatically replies to method calls if errors are returned.

Thus: add an sd_bus_error argument to all message handlers. When we
dispatch a message handler and it returns negative or a set sd_bus_error
we send this as message error back to the client. This means errors
returned by handlers by default are given back to clients instead of
rippling all the way up to the event loop, which is desirable to make
things robust.

As a side-effect we can now easily turn the SELinux checks into normal
function calls, since the method call dispatcher will generate the right
error replies automatically now.

Also, make sure we always pass the error structure to all property and
method handlers as last argument to follow the usual style of passing
variables for return values as last argument.
logind,machined,run: properly invoke StartTransientUnit() bus call
bus: properly serialize unit file change list
core: fix deserialization of StartTransientUnit() parameters
core: fix serialization of client tracker
core: fix serilization of ListJobs() response
core: actually make SwitchRoot() bus call do the deed
build-sys: enable gcc Link Time Optimization when optimization is enabled
timer: make timer accuracy configurable

And make it default to 1min
bus: restore selinux access control to PID 1 for properties
bus: also add error parameter to object find and enumerator callbacks

Just in order to bring things inline with the method and property
callbacks.
bus: add calls to query attached objects
event: rename sd_event_get() to sd_event_source_get_event()
bus: deal with duplicate values from enumerators
cgroups: Cache controller masks and optimize queues.