session vcl: support abstract sockets for app ns

session vcl: support abstract sockets for app ns

App namespaces can now be associated to a linux ip netns, e.g.:

app ns add id <ns_id> secret <n> sw_if_index <n> netns <netns>

If session layer's app sock api is enabled, this triggers the creation
of an abstract listening socket in the netns that has been configured.
For the example above that would be @vpp/session/<ns_id>.

Consequently, vcl, or other apps attaching to vpp, can connect to said
abstract socket from an ip netns without the need to share unix domain
socket files. In particular, for vcl it's enough to set app-socket-api
to @vpp/session/<ns_id> in the conf file.

Type: feature

Signed-off-by: Florin Coras <fcoras@cisco.com>
Change-Id: I26fdc626a760a3f423c5b8be4251623f6e9cd73a