sd-bus: sync kdbus.h (ABI break)

Some comment fixes and header cleanups in kdbus.h, and the task capability
meta information has been factored out to its own struct.
units: tmpfiles-setup-dev - allow unsafe file creation to happen in /dev at boot

This will allow us to mark static device nodes with '!' to indicate that they should only be created at early boot.
man: tmpfiles.d - recommend using b! and c!

We should avoid creating static device nodes at runtime.
shared: in-addr-utils - add default_subnet_mask and default_prefixlen methods

These use the (deprecated) IPv4 address classes to deduce the corresponding subnet masks. This is useful when addresses
without subnet masks and prefix lengths are given.

Make use of these new functions from sd-dhcp-lease.
networkd: network - if no prefixlength is given, try to deduce one from the address class

In case of a class E or F address, ignore the address.