vcl: add support for app socket api

To enable add "app-socket-api /path/to/socket" to vcl.conf. On vpp side, add
use-app-socket-api to session stanza in startup.conf

VPP allocates a socket per application namespace which it places in the
app_ns_sockets subfolder of the run folder (default /var/run/vpp). The
socket used implicitly selects the app namespace for the vcl app.

Type: feature

Signed-off-by: Florin Coras <fcoras@cisco.com>
Change-Id: Ifc14b93dcbf6ef9bed1852d46cd069f4855b92ef