kdbus: check policy after opening, before Hello

kdbus: check policy after opening, before Hello

This moves checking access policy to the point after
the kdbus file descriptor is open, just before "Hello" is performed.
Now, it reflects standard process a bit more.

The standard process is:
- open fd (e.g. socket);
- check authentication, if needed and possible;
- connect to the bus (say hello).

In kdbus, we have only:
- open kdbus fd;
- connect to the bus (ioctl KDBUS_CMD_HELLO).
Calling libdbuspolicy for authentication fits between the two.

Additionally, and most importantly, this is required to share
the connection between gio and libdbuspolicy in the future.

Change-Id: Iee49b36e482a099d061dff4a8ba1826c2a53bb9a