networkd/sd-dhcp-server: only start dhcp server when necessary
sd-dhcp-client/networkd: set lifetimes for IPv4 addresses

Note that /proc/sys/net/ipv4/ip_dynaddr needs to be non-zero.

[tomegun: hook up DHCP renew events to increase the lifetime when necessary]
networkd: merge DHCPv4 and DHCPv6 config

If there are v4 or v6 specific options we can keep those in separate sections,
but for the common options, we will use only one.

Moreovere only use DHCP=[yes/both|no/none|v4|v6] to enable or disable the clients.
Add support for DHCP static route options

This adds support for DHCP options 33 and 121: Static Route and
Classless Static Route. To enable this feature, set UseRoutes=true
in .network file. Returned routes are added to the routing table.
networkd: set static addresses immediately

Don't wait for IPv4LL nor DHCP to finish before setting statically configured addresses.
units: local-fs.target - don't pull in default dependencies

Reported by Gerardo Exequiel Pozzi:

Looks like [commit a4a878d0] also changes a unrelated file
(units/local-fs.target) [partially]reverting the commit
40f862e3 (filesystem targets: disable default dependencies)

The side effect, at least in my case is that the "nofail" option in both
"crypttab" and "fstab" has partial effect does the default timeout
instead of continue normal boot without timeout.