app/testpmd: add flow validate/create commands

app/testpmd: add flow validate/create commands

Syntax:

 flow (validate|create) {port_id}
    [group {group_id}] [priority {level}] [ingress] [egress]
    pattern {item} [/ {item} [...]] / end
    actions {action} [/ {action} [...]] / end

Either check the validity of a flow rule or create it. Any number of
pattern items and actions can be provided in any order. Completion is
available for convenience.

This commit only adds support for the most basic item and action types,
namely:

- END: terminates pattern items and actions lists.
- VOID: item/action filler, no operation.
- INVERT: inverted pattern matching, process packets that do not match.
- PASSTHRU: action that leaves packets up for additional processing by
  subsequent flow rules.

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Olga Shern <olgas@mellanox.com>