lib, zebra, pimd: clean up/fix VRF DECLVAR macros

lib, zebra, pimd: clean up/fix VRF DECLVAR macros

There's a common pattern of "get VRF context for CLI node" here, which
first got a helper macro in zebra that then permeated into pimd.

Unfortunately the pimd copy wasn't quite adjusted correctly and thus
caused two coverity warnings (CID 1517453, CID 1517454).

Fix the PIM one, and clean up by providing a common base macro in
`lib/vty.h`.

Also rename the macros (add `_VRF`) to make more clear what they do.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>