busname: add parser for bus name policies

There are three directives to specify bus name polices in .busname
files:

 * AllowUser [username] [access]
 * AllowGroup [groupname] [access]
 * AllowWorld [access]

Where [access] is one of

 * 'see': The user/group/world is allowed to see a name on the bus
 * 'talk': The user/group/world is allowed to talk to a name
 * 'own': The user/group/world is allowed to own a name

There is no user added yet in this commit.
sd-bus: sync kdbus.h (ABI break)

Bring kdbus.h in sync with recent policy related changes. Most notably,
KDBUS_MAKE_POLICY_OPEN is now gone. The rest doesn't matter for systemd
at this point, as the policy functionality was not yet used.
sd-bus: add support for policy upload on activator connections

Activator connections may upload policy when registering to the bus.
This patch contains code to translate between BusNamePolicy objects and
the kdbus specific items.
sd-bus: add transition hack to open policy

Open the policy as much as possible for names that don't specify policy
in the busname files.