stream-unix: Use rundir as root for relative paths.

stream-unix: Use rundir as root for relative paths.

Until now, "unix:" and "punix:" paths that are not absolute have
been considered relative to the current working directory.  It
is more useful to consider them relative to the rundir, so this
commit makes that change to the C and Python implementations of
the stream code.

This commit also relaxes the whitelist check in the bridge code
so that any name that does not contain a "/" is considered OK.

Signed-off-by: Pavithra Ramesh <paramesh@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>