unix: mkdir VPP_RUN_DIR before opening a socket in it

unix: mkdir VPP_RUN_DIR before opening a socket in it

Change https://gerrit.fd.io/r/#/c/7230/ added a Unix domain
CLI socket in the default startup.conf; however unless you
had previously run VPP with the DPDK plugin enabled the
directory that it is created in. /run/vpp, would not exist
and startup would fail. This directory is typically hosted
in a tmpfs ramdisk and is thus ephemeral.

This patch adds a function that attempts to mkdir VPP_RUN_DIR
and uses it in both the DPDK plugin and the CLI code if the
CLI socket is to be created in that directory.

Change-Id: Ibbf925819099dce2b5eb0fa238b9edca1036d6fd
Signed-off-by: Chris Luke <chrisy@flirble.org>