memif: Add new API calls to manage memif socket names.

memif: Add new API calls to manage memif socket names.

New API calls and corresponding CLI commands allow the user
to manage the socket filenames for memif connections using:

    vppctl# create memif id <u32> filename <socket-filename>
    vppctl# delete memif id <u32>

and then referencing it later in a memif interface:

    vppctl# create memif <u32> socket-id <id> mode <mode> <master|slave> ...

Corresponding VAT cli entries have also been added.

The default memif socket file at id 0 are still always present.
The existing memif create/delete CLI commands have been slightly
altered into the new syntax:

    vppctl# create interface memif ...
    vppctl# delete interface memif ...

Change-Id: If2bdc7eac3d81e1d9011a5869747e52fc5e11639
Signed-off-by: Jon Loeliger <jdl@netgate.com>
Signed-off-by: Damjan Marion <damarion@cisco.com>