tcp builtin_client: bug correction and short_help

The missing call to `vnet_session_enable_disable' would cause segfaults
because of an uninitialized session pointer. Just as in the builtin
server the session needs to be enabled for the client.

The `short_help' of the CLI command was also populated with the
arguments that are accepted in the parsing function
`test_tcp_clients_command_fn'.

Change-Id: I88c16efa80597dd19b406e8cf1c87aec1ec73573
Signed-off-by: Clement Durand <clement.durand@polytechnique.edu>
Signed-off-by: Pierre Pfister <ppfister@cisco.com>