networkd: add some debug info about notifications we ignore
networkd: improve link state change logging
networkd: improve logging

This gives a bit better messages when a link is added twice.
networkd: print the ifindex of added links

This debug information may be useful when comapring to dropped rtnetlink messages.
networkd: link hash uses 64, not 32 bit keys
net-util: add support for Type=ethernet

When DEVTYPE is not set for a nic, it means it is a wired/ethernet
device.
man: networkd - document DHCP option
man: document a batch of bus functions
systemctl: improve readability on failed commands

Not long ago a failed command would print:
"Failed to start something.service: ..."
regardless of whether the command was to start/stop/restart/etc.

With e3e0314 this was improved to print the method used. E.g. for stopping:
"Failed to StopUnit something.service: ..."

This patch matches the method to a more human readable word. E.g:
"Failed to stop something.service: ..."
Revert "net-util: add support for Type=ethernet"

This reverts commit 4cd1214db6cf4b262e8ce6381bc710091b375c96.

This may still be fixed in the kernel, revert this for now until
we see how it all shakes out.
udev: net_id - handle virtio buses

This was already supported in path_id, so should be uncontroversial.
sd-event: add missing symbol, sd_event_source_set_io_fd

This appears to have been omitted in 30caf8f3afd29da8 when the
the function was added.