man: sd_journal_send does nothing when journald is not available

https://bugzilla.redhat.com/show_bug.cgi?id=1096067
networkd: link - clean up state files

Also keep the path to the lease file around rather than regenarating it all the time.
networkd: introduce refcounting for Links and NetDevs
networkd: network - merge all netdev parsing into one function
networkd: network - do reference counting on netdevs
networkd: link - take refcounts on links

We need to take a refcount on the link whenever we expect a callback. The exceptions
are the ipv4ll/dhcp clients as their lifetimes are guaranteed to be shorter than that
of the link.
networkd: netdev - cancel all callbacks when freeing

This notifies the link that the netdev no longer exists.
networkd: netdev - introduce LINGER state and netdev_drop()

We need the LINGER state in case we still have references to the netdev after it has been dropped.
networkd: link - introduce LINGER state and link_drop()

We need the LINGER state in case we still have references to the link after it has been dropped.
networkd: netdev - drop if creation fails

This ensures that all links waiting to be enslaved are notified that the netdev does not exist.
networkd: manager - drop links and netdevs when we receive DELLINK
sd-dhcp-client: improve logging when stopping client

'Requested by user' was confusing, just drop it.
networkd: reorder bonding and bridging

A link should only ever be part of one, but if we accidentally do both, let's do it
in the right order so the failure is more obvious in the logs.
networkd: link - handle links coming back to life

When enslaving devices, we may receieve DELLINK/NEWLINK for the same ifindex,
let's not be confused by this.
networkd: link - don't log errors when missing routes/addresses are dropped

We were ignoring the wrong errno.