memif: autogenerate socket_ids

memif: autogenerate socket_ids

This patch adds an API memif_socket_filename_add_del_v2
that allows autogenerating memif socket_id when passing
~0 in the socket_id field.

It opportunistically walks the hash to find a free ID
to use, and returns it in the reply.

socket_filename also becomes a variable length string,
to accomodate for longer names (in case a netns gets
passed)

Type: feature

Change-Id: I33fc3e1cf553af27579d6bad8691b22b530531cc
Signed-off-by: Nathan Skrzypczak <nathan.skrzypczak@gmail.com>