network: add support for dropping address
networkd: distinguish between static and dynamic addresses/routes

Static addresses/routes are associated with a network. Dynamic
addresses/routes are associtade with links (as the corresponding network
may be shared by several links).
networkd: add DHCPv4 support

This adds basic DHCPv4 support. Link-sense is enabled unconditionally,
but the plan is to make that configurable.

I tested this in a VM with lots of NICs and over wifi in the various
coffee shops I found this Christmas, but more testing would definitely
be appreciated.