zebra, lib: send VRF backend to the client daemons

zebra, lib: send VRF backend to the client daemons

Add a new field in the ZEBRA_CAPABILITIES zapi message specifying
the VRF backend in use.

For simplicity, make the zclient code call vrf_configure_backend()
to apply the received value automatically instead of requiring
the daemons to do that themselves in their zebra_capabilities()
callbacks.

Additionally, call zebra_vrf_update_all() only after sending the
capabilities message to the client, so that it will know which VRF
backend is in use when processing the VRF messages.

This commit fixes a couple of bugs in the "interface" CLI command and
associated northbound callbacks, which behave differently depending
on the VRF backend in use. Before this commit, the vrf_backend
variable would always be set to VRF_BACKEND_NETNS in the client
daemons, even when zebra was started without the --vrfwnetns option.
This could lead to inconsistent behavior and subtle bugs under
specific circumstances.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>