lib: fix binding to a vrf

There are two possible use-cases for the `vrf_bind` function:
- bind socket to an interface in a vrf
- bind socket to a vrf device

For the former case, there's one problem - success is returned when the
interface is not found. In that case, the socket is left unbound without
throwing an error.

For the latter case, there are multiple possible problems:
- If the name is not set, then the socket is left unbound (zebra, vrrp).
- If the name is "default" and there's an interface with that name in the
  default VRF, then the socket is bound to that interface.
- In most daemons, if the router is configured before the VRF is actually
  created, we're trying to open and bind the socket right after the
  daemon receives a VRF registration from zebra. We may not receive the
  VRF-interface registration from zebra yet at that point. Therefore,
  `if_lookup_by_name` fails, and the socket is left unbound.

This commit fixes all the issues and updates the function description.

Suggested-by: Pat Ruddy <pat@voltanet.io>
Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>

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