Factor out S,G entry state checking functions for existence, forwarding,
blocking and timer to lib.sh so they can be later used by MLDv2 tests.
Add brmcast_ suffix to their name to make the relation to the bridge
explicit.

Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>