sd-bus: update to current kernel version, by splitting off the extended KDBUS_ITEM_PI...

sd-bus: update to current kernel version, by splitting off the extended KDBUS_ITEM_PIDS structure from KDBUS_ITEM_CREDS

Also:

- adds support for euid, suid, fsuid, egid, sgid, fsgid fields.

- makes augmentation of creds with data from /proc explicitly
  controllable to give apps better control over this, given that this is
  racy.

- enables augmentation for kdbus connections (previously we only did it
  for dbus1). This is useful since with recent kdbus versions it is
  possible for clients to control the metadata they want to send.

- changes sd_bus_query_sender_privilege() to take the euid of the client
  into consideration, if known

- when we don't have permissions to read augmentation data from /proc,
  don't fail, just don't add the data in