Add a vpp_restart command

Add a vpp_restart command

vpp_restart solves the problem of restarting vpp and not exiting until
the new vpp instance is sufficiently established to allow immediate
binary API connections. The point is to avoid using arbitrary "sleep
N" commands e.g. in CSIT shell scripts.

We send SIGTERM to the current vpp process, and expect / depend on the
process-monitor du jour to remove the vpp-api shared-memory segment.

vpp_restart exits w/ status 0 if all is well, non-zero upon failure.

In trying to make vpp_restart reliable, we discovered a
recently-introduced heap corruption bug in .../dpdk/init.c, which
caused vpp to crash on startup once every 20 times on a particular
rig.

If possible, we should check /var/log/syslog for evidence of
unexpected multiple restarts during regression-testing.

Change-Id: Ic48c74b1a94a4368066ba2e326d4b12d51192088
Signed-off-by: Dave Barach <dbarach@cisco.com>