macro: add a macro to test whether a value is in a specified list

Introduce IN_SET() macro to nicely check whether a value a is one of a
few listed values.

This makes writing this:

        if (a == 1 || a == 7 || a == 8 || a == 9)

nicer, by allowing this:

        if (IN_SET(a, 1, 7, 8, 9))

This is particularly useful for state machine enums.
core: extra paranoia when deserializing kdbus fd
bus: make sure we check for "incompatible" flags negotiated with kernel kdbus
bus: add .busname unit type to implement kdbus-style bus activation
bus: add generator that turns old dbus1 activation files into .busname + .service units