sd-rtnl: introduce read ether and ip address functions

This patch introduces reading ethernet address and IPV4/IPv6
as well which is based on table based look up.

[tomegun: rename read_ether() to read_ether_addr() to match the append function.]
networkd: use new rtnl_message_read() API
busname: don't drop 'service' from the result string
man: reference the Discoverable Partitions Spec from the gpt-auto-generator man page
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.
dbus1-generator: set AllowWorld=own for now
units: set 'AllowUser=root own' and 'AllowWorld=talk' own for all .busname files
Make tables for DEFINE_STRING_TABLE_LOOKUP consistent

Bring some arrays that are used for DEFINE_STRING_TABLE_LOOKUP() in the
same order than the enums they reference.

Also, pass the corresponding _MAX value to the array initalizer where
appropriate.
sd-rtnl: increase default timeout to match sd-bus
networkd: link - degrade failed UP to warning

Something else may still bring the link up, so don't enter failed state prematurely.
gpt-auto-generator: probe only partition table and partitions we are interested in

Instead of iterating through the list of partitions and probing them all
with blkid, simply probe the partition table and use that information to
only probe the partitions we care for.
update TODO
sd-bus: check for potential integer overflow in KDBUS_ITEM_FOREACH()

For large values of item->size, the 'part' pointer can wrap around,
which results in an illegal pointer, but currently passes the for-loop
condition.
networkd: fix confusion from missing braces

Fixup for 76800848f281c3 'networkd: link - degrade failed UP to warning'.
sd-bus: sync kdbus.h
bus-proxyd: add first stub of a DBus-1 xml policy parser