tools: ynl: default to --process-unknown in installed mode

We default to raising an exception when unknown attrs are found
to make sure those are noticed during development.
When YNL CLI is "installed" and used by sysadmins erroring out
is not going to be helpful. It's far more likely the user space
is older than the kernel in that case, than that some attr is
misdefined or missing.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>