kdbus.h: split in and out flags of ioctls (ABI break)

kdbus.h: split in and out flags of ioctls (ABI break)

Instead of negotating kernel flags via the same field for input and
output, use two different bitfields for that, and call the returned
flags 'kernel_flags'.

The approach implemented before didn't turn out to work too well for
more complex userspace programs that retain the same ioctl buffers
for multiple calls, and which had to manually save and restore the
flags before.

While at it, rename conn_flags → flags in kdbus_cmd_hello to ease
the internal helper functions and unify the API a bit more.

Signed-off-by: Daniel Mack <daniel@zonque.org>