lib,vtysh: Use backoff setsockopt option for freebsd

lib,vtysh: Use backoff setsockopt option for freebsd

Commit: 9112fb367b1ae0168b4e7a81f41c2ca621979199

Introduced the idea of setting the socket buffer
send/receive sizes.  BSD's in general have the fun
issue of not allowing nearly as large as a size as
linux.  Since the above commit was developed on linux
and not run on bsd it was never tested.  Modify the
codebase to use the backoff setsockopt that we have
in the code base and use the returned values to allow
us to notice what was set and respond appropriately.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>