We have ignition, we have tremendous fire!

That way we can allocate an error struct on-the-fly while calling a
function. Nice!

Since journal-gatewayd is now running unprivileged, and detecting
virtalization requires privileges, query PID1 via D-Bus for the used
virtualization.

This is also the first time we use libsystemd-bus for more than just
testing.

https://bugs.freedesktop.org/show_bug.cgi?id=62173

This function always returns the server side ID. The name suggested it
was actually always the peer's ID, but that's not correct if the call is
called on a server bus context. Hence, let's correct the name a bit.

This includes code to parse and split up match strings which will also
be useful to calculate bloom filter masks when the time comes.

This way we can hide more than just the hello logic behind this flag,
for example, later on automatic match management.

This alos gets rid of explicit sd_open_fd() and sd_open_address()
constructors in favour of sd_new() + sd_new_start() where the
negotiation parameters may be set it in between.

Instead of allowing certain actions fail during authentication and
connection setup, implicitly synchronize on the connection to be set up
completely before returning.