tree-wide: rework flags negotiation (ABI break)

tree-wide: rework flags negotiation (ABI break)

We are obliged to reject all bits in flags fields that are not known
to the kernel. In order to let userspace know which flags the kernel
knowns about, we agreed to always write back to the flags field in the
ioctl buffer, even if the call succeeded. The kernel will, however,
will always set the KDBUS_FLAG_KERNEL bit, which consequently is always
invalid when submitted by userspace.

Move some checks from other place to handle.c, and update the testsuite
and documentation accordingly.

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