API: Python and Unix domain socket improvement

Handle the case where buffer overflows.
Then SOCK_SEQPACKET assumption that multiple API messages
are not returned by recv() is broken. Use SOCK_STREAM for
API exchanges instead.

Add support for running tests over sockets.
make test SOCKET=1

Change-Id: Ibe5fd69b1bf617de4c7ba6cce0a7c2b3f97a2821
Signed-off-by: Ole Troan <ot@cisco.com>

test/Makefile

@@ -297,6 +297,8 @@ help:
 	@echo ""
 	@echo " SKIP_AARCH64=1       - skip tests that are failing on the ARM platorm in FD.io CI"
 	@echo ""
+	@echo " SOCKET=1             - Communicate with VPP over Unix domain socket instead of SHM"
+	@echo ""
 	@echo "Creating test documentation"
 	@echo " test-doc            - generate documentation for test framework"
 	@echo " test-wipe-doc       - wipe documentation for test framework"