session: add unix socket api for app attachment

session: add unix socket api for app attachment

This is an af_unix socket alternative to the binary api. To enable it,
add use-app-socket-api under session stanza in startup.conf. When the
socket api is enabled, attachments through the binary api are disabled.

The socket api only works with memfd fifo segments, i.e., shm segments
are not supported.

Type: feature

Signed-off-by: Florin Coras <fcoras@cisco.com>
Change-Id: I55ffcee201d004846daeeec85c700c7e7a578d43