feature: add vnet_feature_is_enabled

feature: add vnet_feature_is_enabled

A predicate function which says whether a given sw_if_index has a
certain feature enabled.

int vnet_feature_is_enabled (const char *arc_name,
                             const char *feature_node_name,
                             u32 sw_if_index)

returns 1 if the feature is enabled
returns 0 if the feature is not enabled
returns VNET_API_ERROR_INVALID_VALUE if the arc name is not found
returns VNET_API_ERROR_INVALID_VALUE_2 if the feature node name is not found
returns VNET_API_ERROR_INVALID_SW_IF_INDEX if the sw_if_index is not found

Type: improvement
Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: Id9ed53e2ae9252efc750ae80fd610456b95cefa4