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>